Sections and Categories - still don't understand

Call me stupid, but i don't get it.

Is it the case that any content/articles to a site posted by a member appears in a defined section. So when you click create a page, it lists the sections which articles can be posted to. Categores don't appear here?

A category, categorises the content of the site, so it acts like a container, gathering various content? For example, weekly or monthly articles which is automatically generated.

But how else does content appear in categories? When you create a category, what next? How is content included in it?