This is the act of spreading links to sites, in hopes of getting a higher rank in Google. Free to edit sites, such as wikis and weblogs are hit pretty hard by this.
Google Spammers especially utilize link titles (anchors) for keyword spamming. Wiki's create link titles and parse URL's automatically, so this is very convenient for a spammer. Blogs generally have the ability to insert HTML or hyperlinks into a "comment" post, so search engine spammers use this to increase link popularity and keyword popularity for their web site.