Change how your site looks and the types of pages available for your site to use.
Themes set the overall look and feel of your Concrete CMS site. Themes are presentation focused, and control how your content is presented. Themes can be applied to an entire site through the Dashboard's Themes page, or applied to just a single page (mouse over the Edit icon in your editing toolbar and click Design).
Included in each theme are several Page Templates. Page Templates can be thought of as re-usable visual formats for the various kinds of pages on your site. For instance, you might have a "Left Sidebar" page on your site that features a left-aligned sidebar design. You might also have a Page Template called "Right Sidebar, "Homepage", or "Full Width". Each takes the same blocks you've placed on your page and displays them differently. Like themes, these can be swapped out on-the-fly to change how your page looks with just a few clicks.
Page Types represent the data object of each page. Page Types often contain special backend functionality that makes them unique beyond standard pages. Page Types can have different page templates assigned to them for output. This helps to decouple custom page functionality from page design.
The dashboard Attributes page lets you edit and create the default attributes included in your pages. These control behind-the-scenes page elements like meta tags, as well as concrete-specific attributes that control how your site is displayed by navigation blocks, etc.
Single pages are special pages that do not use one of the typical Page Types. They can used for running custom code and other situations that only need to exist in one particular place on your sitemap.