What is the importance of "Factions" in the novel, Insurgent?

At the age of 16, each member of society must undergo a series of tests and simulations to determine which faction the individual will join. Their membership is based on personality and aptitude. There are five factions that include: Abnegation (selfless), Amity (peaceful), Candor (truthful), Dauntless (brave), and Erudite (intellectual.) Those who can belong to more than one of the factions are known as being Divergent. Both Tris and Tobias are Divergent, as was Tris’ mother even though she officially belonged to the Abnegation faction.

The factions plot against one another to gain power. Tris and Tobias must tread carefully among the groups while they learn that nothing is as it seems. They realize that the factionless (those who are not members of any faction) may be the most dangerous of all because they live in poverty. The factionless are twice the size of the Dauntless.