In Divergent's World like Hunger Games the world has broken into Factions. Each area is responsible for something. You have Candor (the honest) Abnegation (the selfless, and also where our Heroine is from) Amity (the peaceful), Erudite ( the intelligent) and Dauntless (the brave).
Everyone takes a test at the coming of age and they then decide for themselves what fraction they will go live with. Another words you leave your family to go live with the people who are like you and you get to choose... you see your family one more time, but you NO longer live with them, or socialize with them or really do anything in anyway with them. Your new Family is the Faction that you decide to join.
Now Triss is our Heroine, she is from the Faction Abnegation, but she doesn't have to stay there she can transfer, and her choices will surprise you. I don't want to give to much away but I do want to give you the definition of the word Divergent, it means that you can go through the test that make you chose and actually you don't have to choose just one Faction... you belong to all of them and none of them at the same time.