A category or namespace can be thought of as a common subject for a set of pages. The reason for talking about both is that most people will be familiar with the concept of categories while for many previous wiki users namespaces will be immediately clear.
Pages in Doc5 are defined by two things, their category and their pagename. The pagename is like a title, while a category is more like a subject for a collection of pages.
In general, a category is an abstract container providing context for the items (names, or technical terms, or words) it holds and allows disambiguation of items having the same name (residing in different namespaces).
For example, if you wanted to create a group of pages about recipes for baking cakes it would be typical to have one page per recipe. One for chocolate cake, one for banana cake, etc. And an obvious category to use for those pages would be “cakes”. Links between pages would be of the form chocolate or banana.
Categories can be as wide or narrow in their scope as you require. eg. You can create a category for baking, for cakes or for just chocolate cakes.