Pigs, like all other mobs, wander around aimlessly. if you walk up to a pig it will turn its head toward you for a few seconds, then continue wandering when killed pigs drop 0-3 raw porkchops.
Trivia
Pigs can be ridden with a saddle, although you cannot steer.
if lit on fire, a pig will drop cooked porkchops instead of raw.
If struck by lightning a pig can turn into a pig zombie