Blocks are boxes of content rendered into an area, or region, of a web page. The default theme Garland, for example, implements the regions "left sidebar", "right sidebar", "content", "header", and "footer", and a block may appear in any one of these areas. The blocks administration page provides a drag-and-drop interface for assigning a block to a region, and for controlling the order of blocks within regions.
Although blocks are usually generated automatically by modules (like the User login block, for example), administrators can also define custom blocks. Custom blocks have a title, description, and body. The body of the block can be as long as necessary, and can contain content supported by any available input format.
When working with blocks, remember that:
For more information, see the online handbook entry for Block module.