It's based on the book series A Song Of Ice And Fire. It's a fantasy series but there is very little 'fantasy' about it except for the fact that it's in a world that doesn't exist (Westeros) but really it's like a mideval political drama.
It starts off with the Stark family (there are seven main powerful families) and the king (Robert Baratheon) is coming to visit, the hand of the king (your basic 'right hand man' role) has died and Robert is coming to presumably offer the role to Eddard Stark (the head of the Stark family). And you can kind of see how all of this will go from there. There are a LOT of other things going on but without giving away the plot of the entire first episode (yes, I know exactly where the first episode ends, not kidding, the script is online)...that's the set up.
It's so amazing, so so so amazing. I don't think I've ever been obsessed with anything as much as I'm obsessed with it. The only good thing my roommate's girlfriend ever did was introduce us to the series.