Siege Expand makes use of the powerful Siege Tank upgrade to secure an expansion before your opponent. Investing in this tech also sets you up for a stronger mid and late-game army.

Goal
With the Siege Expand your goal is to secure an expansion before your opponent does. After fending off his rush you put yourself ahead economically, an advantage that will grow over time.

10 Supply Depot
12 Barracks
13 Refinery (3 on gas upon completion)
14 Scout
16 Orbital Command + Marine
17 Marine
18 Supply Depot
xx Factory (first 100 gas)
xx Tech Lab on Barracks (next gas)
xx 2nd Refinery
Swap Barracks and Factory
Siege Tank
Siege Mode
Command Centre
(StarPort)

10 Supply Depot.
This build doesn't need a wall-in. Depending on the map you can opt to block the path behind your mineral line to hinder Reaper movement.

Barracks
Build next to or close to your CC to save mining time and to cover more buildings with less turrets in case of Banshees. Make sure an add-on can fit.

Scout
Your scouting is fairly late so you may encounter a wall-in. If you don't, the important thing to see is number of Barracks and type of add-ons. If you encounter a wall-in don't let your SCV get killed but hide it somewhere. Suicide it onto his ramp LATER to check his army composition.

Orbital Command/Marines
Once you get the Orbital Command make sure to use the MULE efficiently. You make the Marines to hold off fast Reapers. You can make 2 to both discourage/hold off a single Reaper and get a Tech Lab on the Barrack in time to finish before the Factory does.

Lift and land the Factory to the Barrack's add-on. If you needed more Marines to hold off Reapers you can make the Tech Lab with the Factory instead.
Build a Tank right away and research Siege Mode as soon as you have enough gas.

Scouting

If you didn't get enough info from your initial scout, the time to scout again is as your tank is building. Run an SCV up to his ramp to check out his composition. If you can't see much, use a scan.
Execution

After getting the Tank and the Siege Mode upgrade, you can usually start building the Command Centre in your main to float it out upon completion. After the Command Centre you add a Starport for Vikings as soon as possible. From here you add a 2nd Fact and 2nd Port, spending your Gas on Tanks and Vikings and your spare minerals on Marines. You transition to a Tank/Viking/Marine mid-game, prioritizing economy over pressure until your scouting or scans indicate superior forces. In the late game you can add a Tech Lab Starport for Banshees (to pick off Tanks) or Ravens for their spells. Make sure to get upgrades from your Armories and keep adding Barracks (+Reactors) for additional Marines with your surplus minerals.

NOTE: You can delay your Command Center on maps where your natural is farther away, requiring more Tanks to effectively cover all the ground.
NOTE: The Marines serves mainly as a Marauder meatshield while your Tanks deal the damage. If your opponent plays a more Tank-heavy style invest in expanding instead, using your Marines as mobile raiding forces or with your Vikings to counter his.
[edit]Adaptation
Based on your scouting you can delay building the Command Center in favor of an earlier Starport or even Engineering Bay. Your opponent has a couple of options.

Fast Banshee
To play safe get a Starport before the Command Center and save up scans. To play really safe get an Engineering Bay before the CC as well.

Fast Drop
Again, you can opt for a Starport or an Engineering Bay before Command Center. Make sure you have a Tank sieged near you mineral line. Keep your Marine and additional Tanks mobile to quickly respond if he drops.

Later Drop
You can CC after the Factory, getting a Starport after without skipping Tanks. Again, keep a Tank sieged in your base and your force mobile between your Main and Nat.

Reaper openings
Marines should be able to fend off Reapers if you place them well. Keep production going, delaying Tank slightly, if you need to. To play safe you can delay your CC until after your 2nd Tank, placing one in siege mode at the ramp and one in your main. Keep your Marines mobile, but one you have your Tanks sieged up, clumped Reapers should go down fairly easily.

Early Infantry pushes
You'll need a bunker. Scout his moving out timing and only build it down your ramp if you feel it's safe. Bunkers are really cost efficient so don't skimp on them

Early Marines/Marauders (before Siege Mode)
Make the bunker and keep Marine production going. Pull SCVs to block the ramp and repair the Tank/Bunker. If he attack with pure Marauder, a rule of thumb is to get his number of Marauders + 1 to repair your Tank. Unsieged Tanks do well vs Marauders so if you can try to focus fire it (without moving it away from the SCVs)

Marines/Marauders (after Siege Mode)
A Sieged Tank on your ramp's splash damage will do wonders versus a clumped group of the moving in. With your Marines and more Tanks coming this should not pose a problem, just make sure not to get caught off-guard and have your Tanks in position in time.

NOTE: On a map like Blistering Sands, his force might try to break down your backdoor. You should have time to get out a second Tank before this time, make sure to Siege it near your Main CC when you spot this.
NOTE: After fending off a first infantry rush you can opt to play greedy and float to a High Yield expansion. It will pay off if you can defend it but it is more risky. Cross or directly adjacent positions on Metalopolis are good for this greedy type of play.
If you fend off his first rush while successfully expanding you are ahead. Make sure to get your expansion's Gas and Orbital Command going and don't be afraid to invest an occasional scan in checking out his army composition.

1 Rax Marauder can be characterized as an aggressive TvP opening for Terran involving Marauders and SCVs followed by a transition to either an FE or a mid game Siege push. It tries to take advantage of a Protoss opening up with Zealots or a fast expansion by quickly enabling Marauder tech.

Goal
The benefit of this build is that when followed explicitly, there is much room and time for attention to detail in the harassment. When properly microed, the scouting SCV provides much needed bonus health to the first Marauder by acting as a shield and distracting the unit AI, making the initial rush very powerful.

10 Supply (1) at choke
@ 100% Supply, Scout
11 Barracks (1) at choke
@ 100% Barracks, Tech Lab (1)
@ 100% Tech Lab:
1 Marauder
Concussion Shells after the OC begins
11 Refinery (1)
13 Orbital Command (1)
@ 100% OC, Calldown: MULE
@ The next 50 energy, Calldown: Supplies
17 Supply (2)
22~24 Barracks (2)
@ 100% Barracks, Marines
24~26 Supply (3)
26~28 One of the following:
Barracks (3) and Stimpack
Factory (1)
@ 100% Factory, swap with Rax Tech Lab
Siege Tanks and Siege Mode
Command Center (2) in main

Notes
Please note that in order to arrive with your first Marauder on time, it is necessary to emphasize that these buildings must be made as soon as possible once the necessary supply is reached.

On a related note: if the Refinery is not made as soon as possible once the first Barracks has begun, it will be necessary to wait for gas in order to build the Tech Lab.
The Calldown: Supply is necessary because in the gap between 13 and 17 supply, the Marauder opening harassment happens and it is imperative to use the 100 minerals to pay for the extra Marauder from the Barracks.

Reinforce your attack with as many Marauders as you can in the early stages of the harassment so that the Protoss will be forced to meet your threat with even more Stalkers, which cost 25 minerals, 25 gas, and 12 seconds more per unit. This will at the very least delay his tech, provided that you present yourself as a proper threat to him.

At 26~28, the choice of Barracks, Factory, or Command Center is determined by whichever you feel is appropriate for entering the mid game. More on choosing which one to go with will be discussed below.

Scouting
The primary thing to watch out for when Scouting the Protoss is the timing on the Cybernetics Core and Stalker:

If the Protoss goes 1 Gate Core, it is likely that he is aware of this opening strategy and will Chrono Boost Stalkers as soon as possible to gain a numerical advantage and discourage containment.
On the other hand, if the Protoss goes 2 Gate Core, it is unlikely that he will be able to meet the Marauder force with enough Stalkers in time once the harassment begins (unless he saved his Nexus energy so that he can Chrono boost both of his Gateways at once).
In the event that the Protoss attempts an FE — in which case he was most likely unaware of your opening because he didn't Scout it in time for whatever reason — the Marauder harassment will be extremely effective due to the lack of Protoss defenses and will therefore most likely result in an early GG.

Execution

The primary goal of the opening phase of this build is to establish an early containment so that it can either be safe to FE or to tech, whether it be to Siege Tanks, in which case it is advisable to push your way into the Protoss main for the win, or Banshees, in which case it is advisable to use them as a way to split or otherwise draw away the Protoss forces — most likely by attacking the mineral line — so that the Marauders can move and tighten the containment.

Micro
The opening phase of this build may demand an intense level of micromanagement in order to properly harrass the Protoss once Stalkers are discovered. Because it is impossible to kite them, as Marauders and Stalkers have the same range, it is advisable to play conservatively once the Protoss reveals his Stalkers until the Concussion Shells have finished researching, at which point it is imperative to chase the Protoss forces deeper into his main. On top of that, the Protoss may build a Zealot to perform the same duty as your SCV: a human shield. Such a situation will make micromanagement that much more involved.

If the Protoss Stalkers outnumber the number of Marauders when Concussion Shells have finished, it is advisable to abandon the harassment and setup a containment whilst still producing Marauders to make up for the lack of numbers. It is unlikely that the Protoss will pressure outward at this point when he realizes that the Concussion Shells will prevent him from retreating without losing too many of his forces. If he notices that he has a signifcant number advantage over your remaining Marauders, though, he will most likely force you to retreat to your base and take map control from you.

Also, please note that it is important to keep the scouting SCV alive — perferably at full health, of course — so that it can serve as a human shield (because the AI will target SCVs before Marauders in many situations if not directly told to attack the Marauders) for the initial Marauder harrassment, thus prolonging the lives of the Marauders and making them more cost-effective. If the scouting SCV dies for whatever reason, it will be necessary to cut an SCV from the mineral line to join the first Marauder when it is finished on the way to the Protoss main.

Lastly, it is a good idea to retreat your Marauders temporarily when reinforcements are arriving from the Barracks so that you can begin the harassment again with even more forces.

Adaptation
When the Protoss becomes aware that this early Marauder harrassment is coming, he will then abandon any economy-centered opening strategy that he may have had and thus be forced to invest in Stalkers (provided that he knows what's good for him) and either try to play aggressive in order to prevent containment before the Concussive Shells have finished or turtle on his ramp and eventually tech to either High Templar, which he will use to chase you away with Psi Storms, or Void Rays, which he will most likely use to harrass your mineral line. In order to know what he's going after your containment has been set up, of course, it will be necessary to use the Orbital Command's Comsat ability on the Protoss base once the initial fighting begins to die down.

Note: If the Protoss is ballsy or is certain that you are not assertive enough to keep him on his toes at all times with the containing forces a Warp Prism drop could also happen, of course, so that you will be forced to call back your containing forces.
If you ComSat the Robotics Facility or the Stargate, though, during the containment, make sure to build the necessary Missle Turrets or Vikings in response
As in Brood War, the most natural choice after completing the initial containment is to expand, in which case this build will reduce to the 1 Rax FE, however the Siege push is also advisable if an early win is desired and the Protoss is far behind going into the mid game, whether it be because of a rather successful Marauder harassment or a militarily weak Protoss player who chose to FE yet somehow held off the initial harassment. In either case, the Marauders are already there and prepared to either maintain the containment or provide a human shield for the Siege Tanks.

If this build has ended up putting you behind (probably because the Protoss cheesed or because you were too aggressive with your Marauders against his fast Stalkers), the most advisable course of action is to switch back to Marines and try to produce a Reaper or two with which to pressure or harrass the Protoss while you try to setup a transition into the mid game and rebuild your forces.

It will be unlikely that an expansion can be made for some time if the Protoss has broken his containment, so a transition to the Siege Expand build, which is frequently used by Tarson when playing against an unknown Protoss build, where roughly 5 Marines and 1 Marauder for every Siege Tank protect an attempt to expand as soon as possible, is advised so that any economic losses due to the lost Marauders in the opening can be recovered as quickly as possible.

Please note, though, that in this scenario, the Protoss will most likely gain map control and force you into a turtling situation... however as Terran this will not be so bad of a position going into the mid game.

... and of course there is always the follow-through mid game strategy of going for a Barracks-heavy build where you simply try to overwhelm the Protoss. After all, there will already be at most 2 Barracks available by the time for the mid game transition.