It seems to be a rule that to write about Divergent, you have to mention Hunger Games, so let's just get this out of the way: Divergent is no Hunger Games. It's what gets made to capitalize on the wild success of Hunger Games, and it will do well partly because of Hunger Games (which I think is an excellent, well-made film).
But Divergent remains good on its own. I was hooked from the first scene. It doesn't feel derivative, though it has a lot in common with its cousin. Both are based on bestselling YA novels that take place in a dystopian future. Both feature strong, intelligent teenage heroines, and both have love stories that take a back seat to the business of overthrowing oppressive regimes. Both also offer biting (if incomplete) critiques of various sorts of political and social problems that confront us today. By my lights, these are all positive things, especially for teenagers, who are plenty smart enough to get it if they're led well.
But while pretty much everyone besides the few wealthy citizens of Panem's Capitol know that the whole spectacle where they make teenagers fight to the death is a bad thing—a sort of fascist-socialist hybrid in which some are very wealthy and others are very, very poor and everyone is under the government's thumb—Divergent's post-apocalyptic Chicago walled-in enclave seems at first to be a happy place, though it's just straight-ahead fascist.
In this society, the "founders" segregated citizens into five "factions," a word that seems to have been stripped of its negative connotations. Factions are a sort of enlightened caste system, with all factions supposedly equal and membership based on what business types would call your "core competency." Your faction defines what you do with your life: service (Abnegation, the public servants), intelligence (Erudite, the scholars), peacefulness (Amity, the farmers), honesty (Candor, the lawyers and judges), and fearlessness (Dauntless, the protectors). Those who belong to no faction are the factionless—poor, hungry, homeless, dirty, to be pitied and feared.
Teenagers take aptitude tests that tell them which faction they're best suited for, and then they're allowed to choose which to join. Many end up in the faction their parents raised them in. Others choose based on their aptitude, or on the one that interests them most. Once the teenager chooses—in a ceremony witnessed by the entire community—they're not allowed to reverse their decision. They've chosen for life.
(A brief aside here: these tests rather clearly correlate with the career tests, and personality tests, and even those "which season/sandwich/celebrity/Disney Princess are you" Buzzfeed quizzes that we youngsters seem to endlessly take which seek to categorize us so we can "find our place" in the world. No accident that this occurs in a story written for teenagers.)
Our heroine, Beatrice (Shailene Woodley), was raised in Abnegation by her loving and competent mother (Ashley Judd) and father (Tony Goldwyn), who is second in command to the leader of the society. But her test results are inconclusive, a point that freaks her tester out enough to make her usher Beatrice out the back door and warn her to tell no one, not even her family. Beatrice has always admired Dauntless, she tells us, but she's as surprised as anyone.