What can I say about this movie................... not much!  Basically, Craven (Mel Gibson), is a Boston police officer (maybe detective or something but you get the idea).  His daughter comes for a visit.  They are walking out of the door of his house, someone on the street yells, "Craven!"  The person on the street shoots and kills his daughter.  Craven spends the rest of the movie trying to figure out who the killer is.
There are a lot of deaths, but they are all shootings, so they don't seem so violent (if that makes sense).  Still, I personally don't think younger children should see it, but that's my opinion.   
Other than that, you have to go see it, and really, YOU HAVE TO GO SEE IT!!!  This movie is a suspense, murder-mystery, conspiracy theory all in one. You won't want to leave to go to the bathroom, and if you do, you'll miss something.  Everything is tied together.  It makes sense as the information is revealed but I don't know that too many people will be able to predict the ending.  If you can, you cheated ;-)