Spam, aka Stupid Pointless Annoying Message, is any form of writing that has no purpose or point to it. Spam ranges from anything from posting total gibberish to posting coherent material that has nothing to do with the current subject. It is never funny, no matter what the writer of it thinks.
Spam is currently outlawed by BZP and is punishable by anything from a simple reprimanding from a moderator to being suspended if the offense continues for long enough.