making one word crappy/garbage/unneeded posts like "Sorry" , "thanks" , "good" , "wow" "sad" is considered spam. Copied answers/replies made in a thread are spam. Post what you feel (your posts must be unique), don't copy answers/replies made by others in a thread, make post which have some sense, be on topic and you'll be fine. Its simple and as easy as that.

P.S. Critically, I'd say short posts aren't spam until they've no sense.