If you’ve ever run a blog, you might see people leave comments on your blog such as “I love your design, how did you do it?” or “Great site! I have bookmarked this amazing site OMG!!!!!”. It’s because of something called Search Engine Optimization (SEO). Because website rankings in search engines depend on links pointing to that site, spammers will leave comments on blogs to generate a link to their site. By leaving a compliment, bloggers may be suckered by the flattery and they may be less likely to delete such comments. However, it’s a spamological arms race. The search engines don’t want spammy search results, so they will take steps to filter out sites that rank higher than they should. So this isn’t a very effective technique nowadays if the link is to a site completely unrelated to the blog. Also, most (though not all) blog software and content management systems will place a “nofollow” attribute onto all the links, so links submitted by spammers won’t count towards search engine results.
The truly devious spammers will create fake blog pages that look like a real blog. (Or they may simply copy a real blog.) And then that fake blog links to their real site. By simply following the link to their fake site, it won’t be entirely obvious that they are compliment spamming your blog. But not everybody is this smart and most compliment spam is easy to spot.