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 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. Also unlike other Drupal content, Blocks are not indexed to be found using Drupal's sitewide search.