Differences between wikis and other kinds of websites
The line between wiki, blog and other kind of website can be blurry. Suppose a person has a static HTML website, and gives a few friends FTP accounts and passwords to let them edit the HTML files. Does that make it a wiki? wikipedia:Wiki says, "While a wiki is a type of content management system, it differs from a blog or most other such systems in that the content is created without any defined owner or leader, and wikis have little implicit structure, allowing structure to emerge according to the needs of the users."
That doesn't even sound like an accurate description, because there always is a defined owner or leader. Also, a static HTML website has little implicit structure too, aside from the constraints of the markup language. MediaWiki and its extensions impose quite a bit of structure on how wikitext is to be interpreted. A wiki can also be configured to make it not much different from another CMS.
Some features commonly implemented by wiki software include the storing of page histories so that edits can be easily reverted, and existence-detecting wikilinks.