You can use categories to define sections of your site and group related posts. The default category is Uncategorized but this can be easily changed in your Settings. Pages cannot be associated with Categories. You can only associate Categories to Posts.
There are two ways to add Categories. They can be added while adding or editing your Post or they can be added via the Categories menu option. Adding Categories while editing your Post will automatically assign those Categories to your Post. Adding Categories using the Categories menu option will simply add them to the list of available Categories.