Xbox 360 Achievements (warning: minor spoilers in link) have posted the official achievement list for Grand Theft Auto IV. There are 50 gamerscore-increasing tasks you must accomplish for a total of 1000 points. The list is what you'd expect: complete all missions, do well on the minigames, and various other tasks that to reach 100% game progress (which itself is an achievement).
There are about ten achievements related to multiplayer, with one particularly interesting task to "Kill a Rockstar developer in a ranked multiplayer match" worth 10 points. Achievement completionists, you may need to boost your stalker skills for this one.