StarCraft II: Starter Edition allows anyone to play the following content for free (all you need is a Battle.net account and Internet connection):

The first four missions of the StarCraft II: Wings of Liberty single-player campaign, including Mar Sara 1-3 and the choice to embark on Tychus’s first mission or Dr. Hansen’s first mission.
The first two Challenges: Tactical Command and Covert Ops.
Access to the terran race in Custom Games and Single-Player vs. AI.
Access to the 4 custom maps

A level cap of 20
A maximum of 10 gold
Trade skills are capped at 100 ranks
Unable to trade via the Auction House, mailbox, or player-to-player
In-game access to public chat channels unavailable; players are limited to communicating using only say, party, or whisper
Characters will be unable to create or join guilds
Characters are not able to send whispers to other characters unless they have been added to the characters' friends' lists or have received a whisper from a character first
Characters will not be able to invite other players into a party
Characters will be unable to disable experience gains
Voice chat is disabled on Starter Edition accounts
Realms experiencing login queues will prioritize players who have full, paid accounts
Starter Edition accounts are not eligible for character transfers
RealID features are disabled on all Starter Edition Accounts