StarCraft II: Starter Edition allows anyone to play the following content for free (all you need is a Battle.net account and Internet connection):
Any campaign progress and achievements you earn while playing StarCraft II: Starter Edition are automatically saved to your Battle.net account, and will carry over should you ever upgrade from the Starter Edition to the full version of StarCraft II: Wings of Liberty.
If you’re thinking of getting into the game or know a friend who’s been interested in joining you in the Koprulu Sector, it’s easier than ever to give the ultimate real-time strategy experience a spin. Just head over to the Starter Edition page here, or log in to an existing Battle.net account.
For more information and a full list of restrictions, check out the StarCraft II: Starter Edition FAQ: http://us.battle.net/sc2/en/blog/3250656