Content Blocks are a reusable piece of content that includes a Category Rotator, Featured Products, HTML, Image Rotator, Product Rotator, and RSS Feed Viewer. You can create as many content blocks as you'd like.
You can create a content block by going into the Hotcakes Administration and finding the "Content Blocks" page in the catalog menu. Once there, give your content block a name on the left side, and then click the Add New Column button. This will then give you the opportunity to specify the type of content the block will be.
Once the type is chosen, you'll have an opportunity to define what content will be in the content block. The options will depend on the type you choose.
Out of the box, content blocks are exposed as a header and footer placement in product and category views. Once created, you can specify the content block to be added to any specific category or product in the respective header and footer setting. The chosen content blocks will then appear before or after the category/product when viewed.
In the example below from the Social Spokes demo site (road bikes category), the header you see above the products is a content block.
However, since content blocks and views essentially fully customized HTML, the content block itself does not need to be always displayed as a header or footer. You can get creative with them to be pop-ups, sidebars, and more.