Hey, I have a question! :DD

There is no difference, like if a crack ship was Katniss/Harry, the slash pairing would also be Katniss/Harry. So basically there is no difference. :) AND, I ship Harry/Katniss now. XDD

I found out what both mean. Slash is a term used, in fandom, for homosexual pairings, that are not meant to be homosexual. Like Harry/Draco are slash and Hermione/Luna are slash too.
Crack pairing is when a pairing is so ridiculous that it seems that the person must be on crack or something to ship them. It's basically like a joke pairing. Ex: Draco/apple, Ginny/Ron, Hermione/Sorting hat. Some people even find some slash pairings ridiculous, so they classify it as a crack pairing too, like Draco/Harry. :)