Posts and Pages are very similar in some ways. They both hold content that is stored in the database and can be displayed by a template on a web page.
The big difference is in how WP handles the two. Posts are not related to one another: there is no parent/child relationship. They are time-stamped when published and normally retrieved according to the time stamp, usually in most-recent first order. An ‘archive’ of posts is just a group of posts that fall into a certain group. The group can be a time frame, or some other grouping.
Pages can be related to one another in a hierarchical manner. A Page can have a ‘parent’ and ‘children’. They are not normally retrieved by the time of publishing.
Categories do not hold content. They are just names that you attach to a Post or Page to allow you to group them. Categories also can have a hierarchical relationship to other categories.
If you need more explanation, I suggest you get a copy of ‘WordPress for Dummies’. I started with that, and it was very helpful.
