Here is a debate that has been brewing for awhile now, What is a Wiki? While there is no a one answer fits all here, one can at least admit that ultimately it is a archive and database. But what does a wiki hold for content? Well, the common answer is a wiki holds articles which forms a encyclopedia. However, as touched on in Sarah Manley's blog: Beyond the Encyclopedia - A Wiki for Everyone, there are wikis that are also about fan fiction and even original content (recipe wikis). Likewise, there are parody wikis which aim to have a good laugh of a franchise or subject. So in many ways anything that is a database can also be a wiki.
The second question of this topic is sometimes even more important than the content itself. Some wiki's claim to be a community type wiki, however to edit there is almost impossible due to high on power admins. Wikia like other wiki farms want open communities for many reasons including to promote healthy active wikis. That being said, some are also a one editor show, and sometimes there are reasons for that too include restrictive edits that discourage activity. So in short, there seem to be three major types: One man edit shows, a group of high editors, or a simple open community. Note, none of the three are wrong per-say if the wiki openly says it somewhere. In some cases, like with a fan fiction wiki it make cents to only really allow people comment to the article instead of edit it.