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 layoutThe visual and structural composition of a page. refers to the visual and structural composition of the page. However, the term layout updateA specific set of XML instructions that determines how the page is constructed. refers to a specific set of XML instructions that determines how the page is constructed.
The XML layout of your Magento Community Edition store is a hierarchical tree of blocks. Some elements appear on every page, and others appear only on specific pages. You can see how these structural blocks are referenced by examining the layout update code for your home page. To do so, simply open your CMS home page in edit mode, and select the Design tab to view the Page Layout section. 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 with a widget. To place a CMS static block of content with a widget, you must identify the page, and location on the page, where you want the block to appear. You can use the Widget tool to place a block on any generic page of your store, including the home page and all CMS pages. However, to place a block in the sidebar of a specific CMS page, you must make a layout update.