It's the self-indulgent part of the process that differenitates id-fic, as defined in Fanlore, the "I'm writing this because I want to and it might be a shame to admit I like this".

Crack fic (also defined here) tends to start from a ridiculous premise, like Duncan MacLeod turns into a penguin, or the characters are stuck in a Harlequin Romance novel, etc. It's not that the fic intends to be funny but it is often is, just by the sheer amount of ridiculous things that get written into the story.

As Sholio mentioned, these are not genres - they are types of labels applied to a fic, sometimes by the author themselves, to categorize what type of story they are and why they got written. Hope this helps!