Blocks are the boxes visible in various area of your Drupal website's screens. Most blocks (e.g., recent forum topics) are generated on-the-fly by various Drupal modules, but they also can be created in the administer blocks area of a Drupal site.
Whether, and where, a given block will appear on a page depends on both the theme enabled and on administrative block settings. Block settings are controlled from the block administration screen, reachable by clicking administer » site building » blocks. From this screen, it is possible to control whether each block is enabled where it will be placed on the page, and control the visibility of blocks on each page.
This link has more recent information about Drupal's Blocks:
Also, here are some videos about blocks: