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"Humor" that relies on shock value, such as sex or violence.
Dirty humor is not real humor but sleaze in disguise. Unfortunately, the massive amount of it in culture has brainwashed the world into thinking that true humor (the kind that doesn't nauseate you) isn't humor at all, and dirty humor is real humor.
Examples of media using dirty humor:
Encyclopedia Dramatica
Mad TV (most of the time)
South Park
Most of Comedy Central's programming.