From AoPSWiki

A wiki is a site that is editable by anyone, simply put.

"Wiki" actually means "free", hence the freedom for any registered, or even unregistered user to edit.

Pros/Cons?

Advantages of a wiki include

• Rapid Information Sharing
• Long, detailed articles
• Multiple opinions voiced

Disadvantages

• Extremely prone to vandalism
• Extremely prone to fallacies

MediaWiki

The software standardized for most large wikis, MediaWiki provides a variety of software making wikis like this one with 4,528 articles up to wikipedia (2,000,000+ articles).