Promo Blocks are blocks of content that can be assigned to specific regions/sections of a page(s). Similar to editing page content you can edit and publish the content of promo blocks on your site. Promo blocks are often located on the homepage for a site, but they can also be assigned to positions within the right sidebar. In the following photo of the Student Activities homepage we can see a promo block in the right sidebar named "Make a Gift". If you are logged in and have editing rights to the block content you will see a small (edit) link below the block which will take you to the edit page for that block with its associated WYSIWYG. One significant difference between editing a promo block and a typical CMS page is that the promo blocks do not utilize moderation states. In other words any changes you make will be live after you click save and the cache for that page has expired. Promo blocks do not currently have a way to sit in a draft state like a full CMS page.
Example promo block on the right with (edit) link below