A wiki (IPA: [ˈwɪ.kiː] or [ˈwiː.kiː] [1]) is a type of website that allows the visitors themselves to easily add, remove and otherwise edit and change some available content, sometimes without the need for registration. This ease of interaction and operation makes a wiki an effective tool for collaborative authoring. The term wiki can also refer to the collaborative software itself (wiki engine) that facilitates the operation of such a website, or to certain specific wiki sites, including the computer science site (an original wiki), WikiWikiWeb, and the online encyclopedias such as Wikipedia. (Source: Wikipedia)

PMI on Wikis in Education
Plus - Create an online classroom in a few minutes, easy to edit, public or private domain, add multimedia.
Minus - usually the free ones are general access - this could be a plus or a minus
Interesting - This is now education - it is multimedia, online, collaborative learning

All your questions answered here - http://www.wikispaces.com/help+teachers#toc8 (includes video tutorials)
http://www.wikispaces.com/ or http://www.wikispaces.com/t/x/teachers100K (link for a wikispace for teachers - no advertising)

Comparing types of Wikis?

Examples of Science Wikis:
A class wiki - manage by the teacher but resources collected by the students - http://biology-stlukes.wikispaces.com/
Student wikis - http://science21-stlukes.wikispaces.com/student+pages

Blogs or Wikis - Which one is for me??