It's a computer game, "lame" some of you may automatically assume. Read why I like it and think it's not lame in my other blog here. This is just to explain what SC2 is to people who are unsure.
You are the commander of a base from which you build an army. You then use your army to kill your opponents army and base, if you do that you win. You start with a base: (there are 3 races/ types to choose from)
You have to collect minerals and gas to build more things.
You then build an army to win the game with.
There is an awful lot of flexibility in what tactics you can use to try and win, You have to chose a balance between getting more bases /money, or a big enough army. You also have the choice to developing better technology to have a more advanced and therefore powerful army.