The strongest, purest form is absolute pacifism, in which a person believes that it's always wrong to use violence against other human beings, even in self-defense or defense of another person. It's a difficult course to follow and relatively few throughout history have been willing to embrace it.
Even one of the famous pacifist leaders of all time, Mahatma Gandhi, acknowledged that while he saw nonviolence as "infinitely superior" to violence, he wasn't against India waging war in self-defense. "I have been repeating over and over again that he who cannot protect himself or his nearest and dearest or their honor by non-violently facing death may and ought to do so by violently dealing with the oppressor. He who can do neither of the two is a burden," Gandhi said.
But there are plenty of other forms of pacifism that take more nuanced views than absolutism. In pragmatic or conditional pacifism, someone is opposed to using violence or waging war in a particular situation — for example, the conflict in Vietnam during the 1960s and 1970s — because he or she believes it's the wrong solution for that particular conflict [source: Cady, BBC]. But that person might think it's OK to use violence in some other context, such as defeating Hitler and the Nazis in World War II.
Another moderate form of pacifism is selective pacifism, in which a person opposes certain types of violence — such as wars using nuclear bombs or other weapons of mass destruction because they are so devastating [source: BBC].
You can also look at pacifism in terms of how pacifists actually carry out their beliefs. Some pacifists have such an aversion to war that they'll refuse to participate in any way, and will endure going to prison or worse rather than serve in the military.
Other pacifists take a less rigid position. They won't pick up a gun, but they'll be willing to serve in some non-violent capacity, such as driving an ambulance or working in a hospital. This position is called active pacifism [source: BBC].