The Page Manager lists the different pages on your site and allows you to navigate to them if no other link is possible. You can access the Page Manager by making sure that Site Builder is enabled and clicking on the Pages link in the menu.
This is a list of all custom pages that you or other administrators have created on the site. Clicking on the page link will take you to that page.
This shows a list of all channel (Forums, Article Sections, Blogs, Social Groups) pages created on the site. Clicking on the page link will take you that page.
When you create a custom page, a new template is created along with it. When you delete the page, the template isn't deleted automatically because it may be used multiple times. Any custom page templates that aren't being used by current pages will be listed here. You can clean up your template list by deleting unused templates.