Structure - An Overview
The Structure defines how all the pieces fit together on the page. Think of the page like a puzzle that can be composed of different types of pieces (blocks):
HTML (web page type code)
Content Preview Blocks (1-3 previews of web pages or the latest story in a blog)
Menu Links (shows a menu of links as defined in Navigation)
Other blocks are not general use, but rather Admin use, such as:
User Block (the login, logout bar at the top)
Blog (only works on blog pages)
Content (shows the content of the web page that has been requested)
The Structure Blocks (pieces) appear in the order they are within each region. So a user goes to a page, the structure puts all the pieces of that page together and presents it to the user.
The Structure Blocks can be turned on/off on a page by editing the block and editing the Visibility tab (no checks at all means it will appear on all pages).