Quite a fascinating movie. I remember watching the hindi copy few years back, and being quite impressed. Had been waiting to get my hands on the original for a while now.
The basic setup is quite simple, a poor delinquent boy is accused of murdering his father. There is circumstantial evidence and some eyewitness testimony go against him too. It looks virtually like an open and shut case, and a jury of 12 men is asked to return an unanimous verdict. The movie is shot almost entirely in the single room where the jury debate the case.
There are a handful of interesting psychological insights one gets as the movie progresses. Of course the first one is, how our own thinking and background color whatever we see and express an opinion on, even when we think we're being objective. Then the interesting bits about our "mob mentality", how a thing as simple as a vote can depend so much on who goes first, whether its secret, etc etc. Then there's the point about how things work out differently in a team where you get different skills, even if initially you thought they were skills that aren't needed. There's quite a list of things which makes it interesting.
Definitely worth a watch!