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Blocks are snippets of text or functionality that usually live outside the main content area of a web site, such as in the left or right sidebars, in the header, in the footer, and so on. If you've ever logged in to a Drupal site or navigated to a Drupal administrative interface, then you've used a block. Block permissions and placement are controlled within the administrative interface, simplifying the work of developers when creating blocks. The block configuration page is located at Administer Site building
Blocks (http://example.com/?q=admin/build/block).