A Game of ThronesThe book ends with the initial skirmish for the throne over. The Lannisters and the Starks became embroiled in a conflict that kicked off with the murder of Jon Arryn and the attempt on Bran's life and spiraled out of control when Cat kidnapped Tyrion. This led to the Lannisters trying to take Riverrun and the nearby lands and Robb smashing their army with his own army for the north. With Renly now bringing together an army in the south, this has left the Lannisters between a rock and a hard place, and marching toward Harenhal.
There are currently three contenders for the crown: Joffrey, the crowned king; Renly, who has declared his own kingship; and Robb, who has been named the king in the north. There is much discussion of why Stannis hasn't done anything, when he should be the king before Renly as the older brother.
The StarksNed. Killed by Payne at Joffrey's order.
Benjen. Missing beyond the wall.
Cat. At Riverrun with Robb.
Robb. Named the King of the North by the people of the north and the River lands. Leading an army from Riverrun.
Jon. At the wall, after having fought off a wight and having resisted (barely) the urge to go south to fight.
Bran. Crippled, but the current ruler at Winterfell. Having mystical and precognitive dreams.
Rickon. At Winterfell, growing increasingly wild.
Sansa. Held by the Lannisters. Still betrothed to Joffrey, who she now hates. Told that she must mouth the right things that he wants to hear or it'll go badly for her. Her wolf, Lady, is dead.
Arya. On the streets of King's Landing. Recently grabbed by a black brother, who's purpose is unknown (but seems positive). Her fencing teacher hasn't been seen since everything went down. Her wolf, Nymeria, is wild somewhere in the center of the Seven Kingdoms.
The LannistersCersei. At King's Landing, sometimes ruling for Joffrey (except when he decides that he's going to do something his way).
Jaime. Captured at Riverrun (in the Whispering Woods).
Joffrey. Sitting the throne, doing stupid things and putting the whole kingdom in uproar.
Tyrion. Sent to King's Landing by his father to take a better hold of Joffrey them his mum and counselors are doing.
Tywin. Leading his shattered army to assault Harrenhol.
OthersDany. Has the small nucleus of her own people and three newborn dragons.
Littlefingers. Sitting on the Small Council in King's Landing. Most recently betrayed Ned when he instead brought the soldiers of King's Landing over to the Lannister side.
I'm going to reread A Clash of Kings when we finish watching season 1 of A Game of Thrones on DVD, then I'll write it up here and have all four books in summary.