WikiSpam is the wiki equivalent of e-mail and blog spam. The spammers simply edit wiki pages, either manually or via the use of software robots, to include advertising links to their sites. WikiSpam often appears as explicit links (e.g. http://example.com); as [bracketed] links, which sometimes are difficult to detect if a spammer replaces the URL in a pre-existing [bracketed] link with the spam link; and as numbered links (e.g. ).
Spammers are not really hoping that you will click on their links, although if you do they are likely to be happier. Their actual goal is to improve their position in Google searches. Wikis are characterized by their open access and willingness to let anyone edit pages. Google measures its PageRank? based on links from one site to another, weighted by the PageRank? of the site linking to the other. Wikis are PageRank? machines, being both massively linked and with hundreds or thousands of pages. These two factors, openness and PageRank?, make wikis a tempting target for spam attacks.
Anyone who can edit the wiki can manually fix vandalism as soon as it happens. To make this recovery process easier, UseModWiki? allows you to overwrite the current (presumably vandalized) version of a page with an older version.