Insurgent is the second book in a series set in a damaged but recognizable future Chicago. People are divided into five factions representing five traits. Teenagers are allowed to switch factions when they turn 16, basically turning their backs on their families, and our protagonist Tris does just that. Most of the first novel, Divergent, covered the training she had to survive in the new faction, where only half the 16-year-olds are allowed to stay. (The rest are kicked out and become “factionless,” homeless and without resources.) The first book closed with a big battle.
I enjoyed the first book but found it a little draggy (see my full review here: http://lookgoodifyoudie.blogspot.com/2011/09/divergent-by-veronica-roth.html ). The second book does not drag but moves at a fast pace, introducing new characters and situations as we get to know better some of the characters who survived the battle in book 1. Insurgent ends with a Big Reveal that does address one of my questions from the first book - how does society get broken into factions?
If you enjoyed The Hunger Games and are looking for another dystopian series, I recommend this series by Veronica Roth. You need to start with book 1, Divergent; this is not a series you can easily start in the middle.