A block is a modular unit of content that can be positioned most anywhere on the page. Blocks can be used to display static information such as text, images, and embedded video, as well as dynamic information.
You can use Magento Community Edition’s content management system to create custom blocks of content without writing any code. Blocks can contain text, images, and even video, and can be assigned to any part of the page layout.
In addition, your store includes a selection of built-in blocks that make it easy to add features and functionality to your store. Some built-in blocks are required components of your store, while others are shopping tools that can be enabled or disabled according to your preference. Many of the built-in blocks can be added to your store with the Widget tool. Blocks can also be defined and positioned by entering XML code in the Layout Update box on the target page.