The Page manager and Panels modules used to be one and the same project, originally created as an alternative to Drupal's block system. A simplified description of the two modules is:
- Page manager is a tool for defining new, arbitrary paths on your Drupal site, and configuring how that page should look and work. It is possible not only to define new paths, but also to override some of the paths defined by other modules. One important part of the Page manager functionality is to collect and send contexts, contextual information that may affect the page.
- Panels is a tool for splitting the main content on a Drupal site into several regions and defining what content each region should contain.
It is possible to use Panels without Page manager, but you will lose most of the reason for using Panels. It is also possible to use Page manager without using Panels, but there are few other modules that may provide page content to Page manager. This is necessary for Page manager to be useful.
This chapter presents only the aspects of Page manager and Panels that are relevant for information structure on your Drupal site.
A good video introduction to Page maager and panels can be found at: http://nodeone.se/en/learn-page-manager (you will need to use the links in the sidebar as the links in the body text are broken).