A wiki is a piece of software that allows for the publication and modification of content on internet or an intranet quickly and easily by an individual or a group of people.
"WikiWiki", which means "quick" in hawaian, was the name chosen by Ward Cunningham when he wrote the first wiki around 1995. The best known advantage of a wiki comes from the fact that, in the simplest expressions of the concept, every user is allowed to edit every page.
The ease of use and simplicity comes from the fact that a wiki is available from anywhere using an internet browser and can be edited simply by clicking on the "edit" button on every page. Thanks to features such as the advanced text editor, new users can get a grasp of the concept in minutes.
The best known wiki is probably Wikipedia, which is an online collaborative encyclopedia.
XWiki itself is more than a wiki: it is a Open-Source Second Generation Wiki.