A ripper is a person who steals graphics and/or code off a persons website without permission, and then will post it on a different website, claiming it to be theirs. They are usually people with no actual graphics/codeing talent, but who want to be in with the 'cool crew'. Some especially idiotic one will post the materials ripped onto the original site. Also, some will modify the graphics and/or code by removing copyright tags and notices, to make it harder for them to be detected.
"Hey, he's ripping my stuff."
"That Johhny guy's a known ripper."