Season 1 neatly covered the events in Book 1 (A Game of Thrones) of A Song of Ice and Fire by George R.R. Martin. I'm sure all of us hope that season 2 will be likewise faithful to the second book- A Clash of Kings. Even if there are substantial differences, I know HBO will produce an amazing second season worthy to watch and remember.
Westeros has screen grabs from the trailer with comments on what we are seeing.