The term crack is used to signify stories that are so strangely insane they can only be written and read under the influence of heavy drugs , i.e. , you'd have to be on crack cocaine in order to think up things like this. Of course after being in fandom for a while, many of these ideas seem perfectly normal.
The relationship between fans and Crackfic, too, is one of a loving addiction.
'Crackfic' is less a genre of fanfiction and more a categorisation based on its premise. 'Crackfic' is fanfiction whose scenario is a marked departure from the reality of the original text (in this case, Supernatural).
Crackfic is a thriving genre in Supernatural fanfiction, arguably because of the permissability permissibility of the canon, wherein these out-of-the-ordinary things happen as a matter of course.
This list was started in 2006, when fandom was small. Obviously the number of fics in these categories now is huge. To find more recent examples visit [[Rec Communities]].