There are 3 campaigns or 'chapters' as Eternal_Paladin put it. Each one is a stand alone game in its own right. They also recently came out with an expansion called Eye of the North (note: you need one of the other 3 to play this). Really it depends on how fast you want to level up. The first one, Prophecies, takes a while to get to level 20 (the level cap). As for the other 2 it can be done really fast. An example: I started a ritualist and in 24 hours i was max level. NightFall is a little slower but still way faster than Prophecies.
Hope this helps