Blocks

Blocks are the boxes visible in the sidebar(s) of your Drupal website. Most of the blocks that you will see (e.g., recent forum topics) are generated on-the-fly by various Drupal modules, but you can also create your own blocks.

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, which you can reach by clicking administer » blocks. From this screen, you will be able to control whether each block is enabled where it will be placed on the page, and which pages it should appear on.

Additional Info

You can find more recent information about Drupal's Blocks at this link

http://drupal.org/getting-started/6/admin/build/block

Also, here are some videos about what you can do with Drupal blocks

Using Blocks by Geoff Hankerson
Drupal Blocks Video on GotDrupal.com