Every page in your Concrete CMS site uses a page type. You can think of the page type as the "brain" of the page. In versions prior to 5.7, the internal functionality of a page and the form factor were joined into one file that was then called a "Page Type". However, over time it became clear that it's important to separate the business logic of a page from the visual element of the page's layout. Therefore, we separated the two into Page Types (the business logic) and Page Templates (the visual presentation).
One great reason for using concrete is that you can easily deploy a consistent look (or many looks) across all your pages -- and swap it out for something else, at a moment's notice.
Currently Installed Page Types
The Dashboard's Page Types page lists every page type currently installed on your site.
When you download additional themes & add-ons from the concrete Marketplace that include page types, they'll be listed here (provided that the add-on or theme developer has taken the time to set them up to automatically install).
Note: This excludes Single Pages, which are a different kind of page that doesn't use a typical page type. Instead, single page content is rendered by a special file (view.php) that is slightly different from a page type.