Splog: A splog is a neologism for "spam blog." A spam blog is a blog which the author uses to promote affiliated websites, increase the search engine listings of associated sites, or to sell links or ads.
Splogs are usually created to increase the backlink portfolio or Pagerank of affiliated websites. They can artificially inflate paid ad impressions from visitors, and/or use the blog as a link outlet to sell links or get new sites indexed.
The actual content of splogs is usually inauthentic content, or stolen from other sites. These kinds of sites are known as "scraper sites," scraping content from other websites. These spam blogs usually contain a high number of links to sites associated with the splog creator, which are often disreputable or otherwise useless websites.
The creators of these sites don't care that their articles or blogs are full of useless gibberish. All that matters is that they are filled with heavily-searched terms, and each time someone visits the site after coming across it in, say, a Google search, the creators get paid from the advertiser each time someone clicks on one of the ads that runs next to their gibberish content.
Splogs are often used in "Google bombing," which is when people create heavily-linked content to rank highly in Google search results for unrelated or off-topic search terms.
The term "splog" was popularized in 2005, when Mark Cuban used it publicly, but it's been used to describe these spam blogs since at least 2003.