Pacifism

Can someone help me understand pacifism?  I'm just a little lost.  First of all, I don't know how extreme the word is.  If you're a pacifist, does that mean that you simply disagree with declarations of war?  Or does it mean that you believe all violence is wrong (including punching a guy in the face who's beating or raping a pregnant woman in the alley)?  Either way, I'm pretty positive I'm not a pacifist (watch Tears of the Sun and try to make a case for it), but I feel like it's one of those where I know a lot of people that I consider to be smarter than me who are pacifists, and that makes me wonder if I should be one.  So I want to be a little more informed.