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Its hard to define a wiki. Basically, it is a software system that allows groups of people to create, edit, and share information via a website. But there is more to it than simple web page editing.
From a consumer perspective, a wiki is a tool that lets people create and edit web pages via a browser. The pages are written using a simplified text format…no word processor required. The advantage to this approach is that new users can become contributors very quickly, and experienced user can do advanced editing with very little work.
From a technology perspective, a wiki is ”the simplest online database that could possibly work”1). Its not a relational database, or even a flatfile database. Either one would imposes too much structure on the information. Instead, a wiki is a freeform text entry system with a fancy text-to-HTML rendering engine…nothing more. Everything else is an add-on.
From a creative perspective, a wiki allows people to express themselves in a stable environment. It frees content creators from the confines of their desktops. It eliminates existing ideas about file ownership. One person is no longer a bottleneck. Everyone can input their ideas into web pages. The entire web is resource for content creators.
From a collaboration perspective, document creation will never be the same again. No more “who has the latest document?” questions. No more offline conversations. No more status updates via email. Instead–all of that can happen in the wiki. Several people can edit a web page at the same time 2). Notifications can be sent automatically when a page changes. And links between pages can be created easily.
From a structural perspective, a wiki does not enforce structure on its users. Anyone can create anything in the wiki, in any location. People are free to create their own page structures. The surprising part is that people naturally seek structure, and will strive to create it when they don't find it. There is a satisfying feeling that comes with organizing something.
From a security perspective, a wiki is a highly customizable tool. A wiki can be open, protected, or private. An open wiki allows users to self-register and edit wherever they like. A protected wiki allows everyone to read, but limits editing to certain groups of authorized users. And a private wiki restricts viewing (and editing) to authorized users. Of course there are other security variations in between these three types, but the security concepts remain the same. A wiki administrator has full control over what can be seen, who can see it, and who can edit it. This is done using a concept called namespaces and namespace security. See my page on the subject for more details.