A scraper site is a website that copies content from another website. Scraper sites may simply pick and choose content found on your website and pass it off as their own or they may duplicate the entire look and feel of your website. The purpose of a scraper site is usually to make money through selling ads and the scraped (copied) content is used as filler and to justify the existence of the website. Scraper sites may also pop up online as a result of bad search engine optimization that you may have done yourself or may have paid someone for!
The danger of a scraper site, aside from stealing your content, is that it duplicates your content which in turn may lower your search engine ranking. Many websites that have been scraped have found that their websites rank lower than the offending website. Unfortunately, there is little that can be done if you find a website scraping your content. However, if the website or webpage was created as part any search engine optimization campaign, it is important to ensure that the look and content of such a site is unique.
Scrapper sites have become such a problem as of late that Google has created an actual Scrapper Site Report Utility that you can use to help Google determine who is scrapping content.