A wiki is a collection of interlinked web pages, any of which can be visited and edited by anyone at any time (collaborative software). The concept and software were invented by Ward Cunningham. You can even edit the page you are reading right now (just click "Edit this page", to the left or on the top of this page). However, if you don't have anything to add or correct here, and you just want to see the wiki in action, edit the Sandbox page instead of this one. Also see the editing FAQ.
When it was first published in 1994, the official Star Trek Encyclopedia quickly became the single most canonical reference source for anything having to do with Star Trek. However, the Encyclopedia has not been updated since October 1999 (going on sixteen years at this point), leaving a great deal of undocumented information. In addition, each publication of the Encyclopedia was unavoidably obsolete before it even hit store shelves – there was always a cut-off date after which no information about the more recent episodes and movies was available. Although most of the knowledge missing from the most recent edition of the Encyclopedia is still available in some form or other on the Web, there is no single, definitive resource where all of that knowledge can be found. Memory Alpha's goal is to become that resource.
Linking to an article in Memory Alpha from outside of the wiki is simple – use the domain name http://memory-alpha.org and the directory location /wiki/ plus the article title. Also, it is a good idea to replace spaces with underscores (_).