"Blocks" are a core feature of Drupal. If you are going to work on a Drupal website, you are going to work with blocks.

Blocks are boxes of content.
Blocks can be used to present anything, so just about all content on Drupal sites — other than main content, and primary/secondary menus — is usually a block.
The content within a block can be:
- Text
- Picture
- Video
- Hyperlink
- Or all of the above
Blocks are similar to "widgets" in other content management systems. Or, you can think of them as Post-It Notes stuck to a piece of paper. Each Post-It Note is a block. It is simple to add it, reposition it, or remove it.
A block can be:
- Sized and styled in a variety of ways.
- Easily moved around your page or website.
- Placed on multiple pages in your website (but you only have to update the information in one place).
Blocks can be displayed in "Regions" (such as the footer or sidebar) of your page. The Block Administration page at "Structure" > "Blocks" lists all of your website's blocks in draggable table rows, so you can just drag them between regions.
Each block can be modified to adjust its appearance, shape, size and position — or which Web pages it appears on.
The size of the block can be:
- Automatically resized
- Full-width
- 3/4-width
- 2/3-width
- 1/2-width
- 1/3-width
- or 1/4-width