Categories are an important wiki idea. The concept of them is quite 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. By the way, this page is in the category CategoryCategory.
On the page of the category, you can describe the category 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(CategorySamples)]] lists all pages in the category CategorySamples
[[FullSearch(r:CategoryRecipes\b -r:CategoryCake\b)]] lists every recipe if it is not about a cake :-)
See HelpOnSearching for further information about the syntax.
Normally, categories have to 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. In a hebrew wiki, you would use page_category_regex = '^Category[A-Z]|^קטגוריה .+'.