The Sitemap page provides a way to get to all content on the site and a way to see all RSS feeds on the site. The "front page" section is for any content that gets published to the front page (very few pages). The "type" section breaks down site content by the kind of page that it is. The "Categories" group pages by the content information.
Next to each page you can see the number of posts in that section. Since posts can be in multiple Categories, the sum of the numbers in Categories is more than the sum in "Types".