The newest trailer handily and concisely explains what the previous trailers danced around, such as what exactly this futuristic society and its factions are about, and why it's so dangerous that Tris (Shailene Woodley) doesn't quite fit in anywhere. There are plenty of snippets that we've seen before, but this time around they're organized into a more explicit narrative.
Of course, the plot device for the "Divergent" series can be a little unwieldy to explain in your typical elevator pitch. In Veronica Roth's books, the society that Tris lives in is rigidly narrowed down to five factions that represent certain ideals and have corresponding jobs. Tris comes from the faction of the Selfless, those who prize self-abnegation above all else. However, her personality test doesn't indicate which faction she is most suited to, so she picks Dauntless, the faction known for its fearlessness. Being Divergent is dangerous to the status quo because Tris is a wild card and can choose her own destiny in a way that people who fit neatly into one faction can't; the fearlessness that comes with being at least partially Dauntless makes it doubly so.
"Divergent" hits theaters on March 21, 2014, and sequels are already planned for 2015 and 2016.