Wikis
The word "wiki" comes from Hawaiian expression "wiki wiki" == to do something quickly. The inventor of the first wiki, Ward Cunningham's wiki http://c2.com from 1995 is still running.
Wikitext is perhaps the quickest way for a human to enter hypertext. There are more than one wikitext in existence but quite naturally the one used in Mediawiki, the software that runs Wikipedia and many other wikis.
How to read wiki In mediawiki an external URL will have a small symbol after it to indicate a link not internal to the wiki or reached via the interwiki mechanism.
Notice that internal links and interwiki links have slightly different color with the interwiki links being lighter in hue. If unable to tell just hover over the link and see where it leads to.
A red link means that the target of the link does not exist in the wiki in question.
How to edit wiki?
Use [[w:Fairtrade]] to link to Wikipedia's article on fairtrade and [[w:fr:Commerce équitable]] to link to the article in the French Wikipedia as simple as that. ( works for all languages, just insert the correct lang code )
This wiki has InstantCommons activated so all Wikimedia Commons images can be used on this wiki with the normal syntax: [[File:Filename.png|thumb|right|100px|Caption text]]
Wikipedia tutorial pretty much explains how to edit wikitext.