The less you know about this film, the more you will enjoy it. It depicts a historic event that, while probably very well known in Latin America, is unheard of in Europe. Like a M. Night Shyamalan film, if you read about it first you'll spoil the twist and ruin the surprise ending. Sadly, spoilers for this film is all over the place, even on the back cover of the dvd. So don't look at it.
Suffice to say that it's a prison film, and that it equals, if not surpasses in quality The Shawshank Redemption (long standing nr. 1 on IMDB's list of greatest films in the world). You could almost think of it as a "what happened after" City of God. Yeah, it's that good.