Categories are one way to organize a wiki. There's a special feature in most wikis (and also in this one) that allows to search for backlinks - follow the link of the page title, and you'll get a listing of all pages that link here. This can be used to add a category to pages. If, for example, every page that includes useful code snippets includes a link to category-code, it's easy to find every page with such snippets by just following the link of the title of that category page. This way, pages are less likely to get lost.
Once pages are "lost" e.g. there is no category to put them in you can place them in one. If there is a page without a category try first and see if its linked on any of these pages. Usually it has been categorized if there is a list. If you can see a bunch of pages that have no category you can create a category. A category is basically a page that has link's and descriptions of linked pages and has a link on this page. A page can fall under more than one category. To see the category that the page falls under there is a line and then a category-foo link. Depending on the foo's that are there that is which category(s) the page falls under. (You can have a page that is under more than one category). An example of this is object-systems
This page is a collection of all category pages, so, if you open a new category, just add a link to this in it (and if you find the time, also add it to the list below). This list differs from the list on Community-Scheme-Wiki in that this list is exhaustive, whereas the list on the top page is meant to include the useful starting points.