Pier is a content management system that allows users to manage their content from the browser.
The central concept in Pier is the one of Structure. For example, the current page is a structure. The Blog is also a structure, but it is slightly different than a page. A file is also a structure.
Commands can be executed on a Structure. For example, a Structure can be edited. In the current setup you can edit this site after logging in (the commands appear in the footer). Once in edit mode, you can use the Syntax. To make life easier, Pier also offers access to these commands via shortcuts.
Any Structure can be Viewed. The default View is the one that renders the structure for normal usage. Another example, is the Browse View which offers a file system like view on the Pier structures.
Any Structure has an environment which defines the template. For example, the environment of this page is Templates (you have to be logged in to see it). To set the environment to be a different page use the Settings command. To edit the environment use the Edit Design command.
A Structure can contain Widgets. For example, the environment of this page contains a menu widget on top, and a widget that enumerates the children of the current sturcture to the right.
More about Pier can be found at: http://www.piercms.com.