A wiki is just an easy way of creating, sharing and storing information on the web. Wikipedia is a fine example of a wiki. I have used wikis in a few different ways to help my gaming.
At one point I used to build social systems for NWN persistent-worlds, which were a built as a collaboration of people all over the (real) world. In that case we utilised the co-operative nature of wikis. One of us would start a wiki page and the others would add their bits to the page until we had the whole picture. We also used bulletin boards hosted on the same site to facilitate discussions between us - but that is pretty much the equivalent of an RPoL game thread or private message. So the Wiki worked as a design tool. This is one that harks from a few years back
http://eots-pw.wikidot.com/home
I started supporting my tabletop and RPoL games with digital material some years ago - and kept that on a website. I had maps, descriptions of area, details of what you could buy in the store - and it developed as the game went on. I didn't document a town until the party went there - so it was all new to them (and me half the time :) However, by the time they went back a second time I gad published notes about the place - and they knew what to expect. It helps me keep my game world consistent, and means my players don't have to try and remember things from (real life) months ago. In this version most of the work is mine, with less input from others - so it is more like a website than a co-operative wiki.
This is an example of a wiki that supported a tabletop game -
http://new-haven.wikidot.com/
This one supported my 3e D&D world and was used for two different games on RPoL
http://urgon.wikidot.com/
I got fed up with all these different wikis and started to pull everything together by in a single Game Wiki covering my house rules, acting as a hub for other games material and supporting games that I run. TBH, it doesn't work as well as I would like it to - mainly because I didn't really know what I really wanted at the time. It is due for a revamp / redesign soon :) his one is just a straight website replacement, with a forum. As you will see, the forum effectively finished when the game finished and it has become pure website :)
http://johnbs-games.wikidot.com/
And I am sure there are many other ways that other people use the