Right and left handed pitchers, what's the difference?

OK, aside from the obvious that a righty throws with his right hand and a lefty with his left, what is the difference between the pitches? You always hear that the best lineups have at least one good lefty. The ball seems to end up in the same place, the catchers mitt or it is hit. So why is a good lefty always a "must have" on a starting line up? Any information you give me on this will be appreciated.