A wiki is a website on which users add and edit pages, together, using nothing but a web browser.
Users add and edit pages. This means that the structure of the site itself is also maintained by users. Whether you want to list the session summaries on the main page, or whether you want to link to the list of player characters, or the magic items the party has or knows about, or whether to start with a map and link to the known locations, it’s up to you.
Together. A single person can start a wiki and other players can join in and drop out over time, no hassle. Many wikis I see are basically run by one person. Perhaps this specialisation is a natural thing. One person is the mapper. One person knows the rules better than anybody else. One person writes session notes for the wiki. But it doesn’t have to be that way. A wiki is not a blog run by a single person. Anybody can join.
Nothing but a web browser. We don’t need Google accounts, we don’t need MS Word, we don’t send emails. All we need is a browser. Traditionally, wikis were very simple and Campaign Wiki still is, in many respects. We use markup to format text:
[[Link to a page]], and so on. For those of us who are used to writing on forums, we also support bbCode like
[url=http://paizo.com/pathfinderRPG/prd/]Pathfinder Reference Document[/url].
There are challenges, too. On Campaign Wiki, I assume that the people running the wiki don’t always have the time and patience to set up accounts for their fellow players. That is why you don’t need one. Anybody can join and help out. This also means that in order to spot and undo vandalism and spam, we need to watch Recent Changes for our wikis, the list of edits made. Campaign Wiki maintains a list of recent changes for every wiki, and a big overall list of recent changes for people like me.
If you’re interested in a one page summary, see How wiki works.