Far Cry 3 is an open-world shooter through and through. The setup is simple: You're set loose on a massive island and tasked with gradually conquering it, one dead pirate/tiger/shark at a time. Here's a gun. Have fun. There's also a story you can play through, though the distinction between the "story parts" of the game and the "non-story parts" is an important one: It's in the balance between the two that Far Cry 3 finds success.
The game casts you as Jason Brody, a twentysomething layabout whose drunken island vacation is interrupted by pirates (the scary modern-day kind). They kidnap him, his brothers, and his friends, and set to ransom them and sell them into slavery. The story parts are a long series of mostly linear, welcomely varied adventures Jason undertakes in service of this rescue/revenge plot. The non-story parts, on the other hand, are the emergent action that happens all over the island between missions. As with many of the best open-world games, the story parts are fun, but the non-story parts are what make Far Cry 3 special.