(This probably should go into the main MW2 thread but eh, whatever)
It's a little confusing if you're too used to how CoD4 categorized things.
Basically, you have a primary weapon (Rifle/SMG/LMG/Scope/Riot Shield) which decides what 'class' you are (skin, movespeed). Your primary weapon can have 1 mod, which includes, rapid fire, FMJ (penetration), more mags, silencer, Akimbo (SMGs), and the various scopes (there are a shit ton of them now!). These are all unlocked by using the weapon.
You can select one secondary weapon, from Machine Pistol, Pistol, Shotgun, and Launcher. They also have various mods available to them. (Moving shotties to secondary weapon is a good design decision btw!) Pistols have a mod called "tactical knife" which basically allows you to knife really fast with your pistol out.
You can select one type of 'equipment', from grenades, Semtex, throwing knife, claymore, C4, etc.
You can select one type of special grenade from stun, flash, smoke
That's it for loadout. Now you have the 3 perk slots with 16 total choice (Each slot has a choice of certain types of perks, just like in CoD4)
The difference is each perk, after some use, upgrades to 'gosu' status. e.g. Marathon (EC from CoD4, but unlimited) allows you to climb faster, Iron Lungs has been baked into Steady Aim's gosu status, Sleight of Hand gosu makes you aim down sight faster, etc etc etc. One of the Slot1 perks is Bling, which effectively turns your Slot 1 perk into an additional weapon mod (The exact perk reads "2 attachments for Primary Weapon") meaning you can for example akimbo P90's which also have rapid fire.
This makes for some really interesting combinations. My
ninja time mini movie for example is Marathon + Cold Blooded + Ninja with a tactical knife mod on USP, which means I have unlimited sprint, climb like a monkey, don't appear on UAV/air support, am completely silent, and enemies don't see a red name tag if they mouseover me.
You can set up your own killstreaks (choose 3 out of a bunch) which includes the old UAV/Airstrike/Chopper and adds a lot on top of that, all the way up to a Tac Nuke which wins the game instantly.
You also get a Death Streak which is basically a bonus if you die too many times in a row. Juggernaut and Martyrdom are now Death Streaks, resulting in much less martyr spam. Jugger only lasts a few seconds after spawn.
Hope that helps.