OpenStreetMap's data is stored in a custom-built system, but all the documentation for the project is stored in a separate wiki, using MediaWiki—the software used to run Wikipedia. It's hosted at http://wiki.openstreetmap.org/. In general, any information that isn't part of the map data itself is kept here.
The wiki hasn't been integrated into the login system for openstreetmap.org, so if you want to edit the wiki, you'll need to create a separate account. This isn't necessary if you only want to read the wiki.
You can use the wiki to record any information that you think might be useful to other mappers. This includes definitions of what particular tags mean in OpenStreetMap, details of how well-mapped cities, towns and villages are, instructions for installing and using various software packages, and reviews of GPS receivers.
The wiki should usually be your first resort when looking for the solution to a problem. The easiest way to find information is by using...