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 a story you can play through, but it's in the balance between the "story parts" and the "non-story parts" that Far Cry 3 finds success. 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 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.