What, exactly, is a scraper site? I've heard it used with negative connotation, but I have no idea what it actually is.
A site that uses automated software to copy another site, basically makes a mirror site to make adsense money.
They usually copy snippets from the SERPs, put that together, sometimes deactivate all the outgoing links and viola - they have a lot of content that they can monetise with Adsense. They used to get millions of visitors from SERPs and it used to pay big money but it's not very profitable of late. Google have at least cleaned up the SERPs a bit and scrapers tend not to rank as well as they used to.