What exactly does "fap" mean? [NSFW]

Sorry for the ridiculous question, but I can't understand the difference between fap and masturbation. Does fap mean the whole progress?

For the sake of giving this a fun answer in case you're asking in earnest...

Fap is an internet neologism, an onomatopoeic intransitive verb referring to the act of male masturbation, whose sound is imitative of the associated slapping of testicles against the body. Fapping refers exclusively to male masturbation; female masturbation is (in context and by association, but very rarely in practice) referred to as paffing or squishing. Reportedly it has its origins in the Japanese-language comic sound effects used in manga, in reference to the same act.

'fap' is a slang term. We have many slang terms in English, some more familiar (or crude) than others, for masturbation. They all signify the general, uh, "process," if you will.