Drupal Theme Components: Blocks, Regions, and Menus

By Lynn Beighley and Seamus Bellamy from Drupal For Dummies, 2nd Edition

Drupal themes can be customized partly through the modification of their Blocks, Regions and Menus components. A theme can be modified, for example, to put the primary menu on the left side of the web page rather than across the top. You need to understand three Drupal component terms:

Region: A location on your web page. For example, the Header region is located at the top of all your site pages. It usually contains a logo and the title of your site.
Block: A container that holds a chunk of code. Drupal organizes menus and other chunks of code into blocks. This makes them easier to move around as you redesign the look of your site.
Menu: If you’ve spent any amount of time on the Internet, you probably have a pretty good idea of what a menu is: a set of links to web pages. In Drupal, that basically describes a menu — but those links can also point to nodes (a fancy word for Drupal content) such as articles and basic pages.

When a link to an article or basic page node is chosen from a menu, a web page opens with the article or basic page node content presented as though it were an actual web page.

Think of it this way — menus are stored in blocks, and blocks are placed into specific regions on your website.