PageRank
Official explanation of Google's PageRank system.
Basically, Google ranks results for its search engine by deciding not only how relevant a given result is, but also by looking at the site's PageRank. Determining a page's PageRank involves analyzing both the popularity of a given page (i.e. how often other sites link to it) and how "important and high-quality" the sites are that link to that page (i.e. how high their PageRank is). A page with higher PageRank will appear higher-up in the results than other sites when people search for a certain thing using Google, and thus high PageRank is very desirable for people who want to sell services over the Internet.
Spammers, being the twisted geniuses that they are, have figured out that (a) wikis tend to have very high PageRank, (b) it is very easy for them to put loads of links on wikis, and (c) by putting links to their sites on a wiki, they will improve their own sites' PageRank. That's the motivation behind WikiSpam -- spammers don't actually expect you to click on their links; they just want their pages to be Ranked higher because they've been linked to from a highly Ranked site. (Incidentally, this is generally the same reason that blogs get so-called "comment spam".)
ExaltedWiki has implemented a patch that removes the PageRank usefulness for links on the wiki. This hopefully discourages spammers, but a lot of spammers are too dumb to notice and spam anyway.
See also:
~ Shataina
