A spam blog, sometimes referred to by the neologism splog, is a blog which the author uses to promote affiliated websites, to increase the search engine rankings of associated sites or to simply sell links/ads.
The purpose of a splog can be to increase the PageRank or backlink portfolio of affiliate websites, to artificially inflate paid ad impressions from visitors (see MFA-blogs), and/or use the blog as a link outlet to sell links or get new sites indexed. Spam blogs are usually a type of scraper site, where content is often either inauthentic text or merely stolen (see blog scraping) from other websites. These blogs usually contain a high number of links to sites associated with the splog creator which are often disreputable or otherwise useless websites.
There is frequent confusion between the terms "splog" and "spam in blogs". Splogs are blogs where the articles are fake, and are only created for search engine spamming. To spam in blogs, conversely, is to include random comments on the blogs of innocent bystanders, in which spammers take advantage of a site's ability to allow visitors to post comments that may include links. In fact, one of the earliest uses of the term "splog" referred to the latter.
This is used often in conjunction with other spamming techniques, including spings.
Read more about Spam Blog: History