A vandal is someone reducing the generally perceived quality of articles, usually for fun, or inserting outright nonsense into discussions, erasing articles wholesale, or otherwise disrespecting more effective contributors. This sparks the Wikipedia Vicious Cycle which drives those contributors out.
Overlap with other classifications
Trolls may or may not qualify as vandals. Some think they are social vandals. These tend to be the same people that believe that a Wikimedia wiki is a sort of community.
Wiki Spammers are a type of vandal who add unwanted external links to wiki articles in order to promote their websites (or their customer's websites) in search engine rankings. MediaWiki has some Anti-spam Features. In particular, most new MediaWiki installations use the 'rel=nofollow' attribute on all external links, meaning that spamming those wikis is a completely pointless thing to do!