Blocks are boxes of content that can be displayed within regions of a page (like header, sidebars, footer, etc.). Blocks may be generated by a Drupal module (like a mini-calendar) or you may create your own custom blocks.
Unlike Articles, Basic Pages, Events, Photo Albums/Galleries, and other types of content, blocks do not generate a new page on your website and do not receive their own URL. Blocks can be configured to display on one or more pages and have settings to specify the pages, roles, and/or content types for which it will be visible.
Additional block documentation on Drupal.org: Managing Blocks