What's a Wiki?

A Wiki is a web page that is editable by users.

It's actually a Hawaiian word meaning "fast", used by the programmer who wrote the first wiki software perhaps to indicate that it was a fast way of keeping content current.

Wikis are used in a variety of contexts, most notably in the online resource Wikipedia. Lots of more limited wikis exist -- for instance, the documentation for the Perl script package BioPerl is now operated solely as a Wiki. Open-source code is available, so people can run wikis on their own servers. Lots of wikis are probably used internally in companies, etc. and are not publicly available.