A wiki is a piece of software, usually on a website that does (at least) 2 things:
This wiki is devoted to the discussion of all facets of the x0xb0x, a Do It Yourself Synthesizer based on the TB-303. It also branches out into the general areas of electronics and music. For more information, see WikiMission.
When you want to make a change to a page, it is pretty simple. Click on the "Edit This Page" button right underneath the title. Most wikis have a special syntax for the text that you edit, versus the text that you see, but in most cases the two are very similar. For an example, click on the "Edit This Page" button now.
Links between wikipages are created by surrounding words inside of 2 square brackets [[Like This]] or using something called CamelCase, which are words that are CapitalizedAndSmashedTogetherLikeThis.
If you want to get involved, one of the best ways to do so is to introduce yourself! The best way to do that is to first add yourself to the users page, and create a new wikilink there to your name/alias. Go ahead and click the users link now. There will be simple instructions on how to add yourself to the list on that page.
This particular wiki is hosted on another wiki called BunnyWiki. One of the projects there is to provide some good documentation for the software that runs this particular wiki. It is a work in progress, but it is worth checking out: DokuMetaWiki.
You can also visit the software project homepage: http://wiki.splitbrain.org/wiki