A wiki is a website that allows users to easily create web pages and edit pages others have created.
Wiki’s are based on the use of software that makes it very simple to create such web pages.
Furthermore, the software has features that makes it excellent for collaboration, so anybody can contribute to web pages. It is not necessary that they are members of any particular group - they can contribute without any formalities.
There is plenty of information on Wikis on the Central Wikia website - a useful entry is that of the Help system
The word Wiki is explained in the free encyclopedia Wikipedia - which itself is a prominent example of a wiki.