A wiki is an editable text-based website. But you don't need to understand any complex HTML, or anything like that to use a wiki. A wiki is really only a little bit more complex than a word-processor, but MUCH more powerful, because instead of only being 1 page, it can be lots of pages, all linked up together.
I'll show you how easy it is to use a wiki later, but first I'd like to tell you why wikis are so useful.
Wikis are useful, because they let hidden wiki setup any data you want, in any way that you
want. Rather than constrain you with a specific way of structuring things, you can
link things together in any way that you want. This means you can set things up as
simple, or as complex as you like.
Because they are so flexible, there are literally millions of uses for wikis. Some uses
Keeping a daily journal
Managing investment properties
Implementing a GTD system (As described by David Allen)
Running a business.
Some wikis also let multiple users share an edit the same data. An interesting
example of this online is the Wiki Encyclopedia where a group of people are collaborating to
make a free online encyclopedia.
Perhaps the best way to show how a wiki works is with an example.
Let's say you have a page, called "HomePage" that says "Hello, this is my first wiki