Categories are an important feature on wikis. Creating a category is easy: you just have to write a category name onto the page and you are done. MoinMoin even simplifies this by letting you choose from a list of available categories while you are editing the page.
On the page of the category, you can give a brief description of the category's contents. If you want to get a list of all pages in that particular category, either click the title of the category or insert a suitable macro which lists the pages:
<<FullSearch(category:CategorySamples)>> lists all pages in the category CategorySamples (alternative you can use <<FullSearchCached(@PAGE@)>>) (FullSearchCached caches the search results and therefore displays a category page quicker and also produces less load for the wiki server, for more information please read HelpOnMacros)
<<FullSearch(category:CategoryRecipes -category:CategoryCake)>> lists every recipe if it is not about a cake
See HelpOnSearching for further information about the syntax.
Normally, categories have to be named like CategorySomething. If you want to allow other patterns like CatégorieGaffeur (that is French by the way), then the administrator needs to change the configuration option page_category_regex according to the users' language, see HelpOnConfiguration for configuration tips and ConfigMarket for configuration presets.
The best choice is to create category pages from a CategoryTemplate (see HelpOnTemplates).