A wiki is an editable web site. Anybody can contribute. That’s it. 
Now go back to the HowTo.
If you want to fool around, visit the test wiki:
You can also use the SandBox.
Wikis, In General
- You can also read more about it on Oddmuse:What Is A Wiki.
- explains why wikis (hopefully) generate interesting content and why there is not much abuse.
- Oddmuse:SiteMap is the homepage of the wiki engine – the CGI script (in Perl, I am sorry to say) that produces the pages you are reading.
- See TextFormattingRules for the basic formatting rules you need.