Each page in your Magento store uses a skeleton layout; these are typically one of the following:
One-column layout
Two-column layout with a right-hand sidebar
Two-column layout with a left-hand sidebar
Three-column layout
Some pages may have specific templates assigned to them (for example, your one-page checkout may use the one-column layout while pages created through Magento's CMS tool may use a two-column layout with left sidebar layout), but pages that are not specifically given a layout inherit the default page layout.
There are two types of blocks within Magento:
Structural blocks: These blocks provide regions that Magento can assign content blocks into. These structural blocks act as a skeleton for your store's content, and typically include the header, footer, content, and sidebar blocks.
Content blocks: These blocks provide reusable blocks of content that are populated as required. Examples of content...