Before you begin working with custom layout updates, it is important to understand how the pages of your store are constructed, and the difference between the terms layout and layout update. The term layout refers to the visual and structural composition of the page. However, the term layout update refers to a specific set of XML instructions that determines how the page is constructed.
The XML layout is a hierarchical tree of all visual blocks, which make up the visual presentation of the page. Some elements appear on every page, and others appear only on specific pages. If you have a basic knowledge of XML and experience using the Community or Enterprise editions of Magento, you may already be familiar with the concept of making layout updates and using block references. To learn more about the syntax required for XML instructions, see:
You can see how blocks are referenced by examining the layout update code for your home page. Depending on the theme, it might contain instructions to remove blocks, unset blocks, and add blocks by referencing specific areas of the page layout.
In many cases, the same result can be achieved by either entering the layout update code in XML, or by using a frontend app. When using a frontend app to place a block of content, it’s simply a matter of identifying the page, and location on the page, where you want the block to appear. Frontend apps can be applied to the generic pages of a Magento Go store, including your home page, and all CMS pages as a group. However, if you want to place a block in the sidebar of a specific CMS page, the layout update must be made with XML code.