Most of you won't know what a Wiki is, and even more of you probably won't ever find out.
A Wiki is, in the simplest terms, a web application for organizing documents that is usually open for editing by all. You can start here if you want to learn more about them. There are various Wikis written in everything from PHP to Ruby to PERL. After installing many Wikis over the years, I finally feel like I found one that is very nearly perfect for my personal notets. I say "nearly" because it's still sub 1.0 and has a few important features to be added.
Instiki is written in Ruby and shares the simplicity of that language. It also includes Markdown formatting which makes teaching someone how to mark-up pages a breeze.
Instiki seems to pride itself on the "there's no third step" simplicity of installation, and it's true. I was able to get it running on my Mac, Linux, and FreeBSD machines in only two steps (download, run script). I customized it further by running it through ProxyPass in Apache so I get nice clean URLs, but that's not necessary, it's just nice.
If you had considered using Wikis before, but couldn't get past the installation or the odd formatting requirements, I suggest you check out Instiki.