Blocks are the boxes of content that can be displayed in regions (such as footer or sidebar) on your page. Depending on your themes regions you may have the ability to include blocks in your right, left, middle or any other configurable region of your theme. Many of the modules like ecommerce, polls come with predefined blocks that you can place in any region of your theme. You can create your own custom blocks with any html editor and place these contents in a block. Each custom block has a title, a description, and a body.
Blocks are configured per theme.If you change the theme or enable multiple themes, you will need to configure blocks per theme. Blocks can be enabled or disabled by clicking on Administration > Site Building > Blocks.
Adding a new block in Drupal :
1. From the Blocks overview page (Administration > Site Building > Blocks), click the “Add Block” tab.
2. Insert a Block description (*required), which will be displayed on the block overview page.
3. Insert a Title for the block.
4. Insert the body content for the block. Be sure to confirm you are using the correct input format for your block.
5. (optional) Set User, Role, and Page specific visibility settings.
6. When you click save, the block is created but not placed in a region by default. You will need to drag and drop it into a region in order for it to show up on the site.
Configuring a block in Drupal :
1. Go to the Blocks overview page (Administration > Site Building > Blocks) to configure the blocks on your site.
2. This page allows you to drag-and-drop blocks into different regions as well as reorder the blocks within a region.
3. Next to each block you can click “Configure” to change the specific block settings:
* Block title: Allows you to override the default title for the block
* User, Role, and Page specific visibility: Adjust visibility for block based on user, role or page.
* Additional settings may also be available, depending on the particular block.
Reference : http://drupal.org/getting-started/6/admin/build/block