Spam is any useless shit that doesn't really contribute all that much, or anything to a thread.

examples of which are:
LOL .
HAHA .
J00 are so stupid.

If i made a thread about garfield and you posted I LOVE ANIMANIACS, that's spam because it has nothing towards what I have to say.

Replying to someone saying "haha that's funny" or some shit is also spam.

Getting a basic idea?