A custom block contains content supplied by you (as opposed to being generated by a module). Custom content may be either static (i.e., HTML) or dynamic (PHP generated content). Virtually all of the functionality of Drupal is accessible from within a PHP content block. The flexibility of blocks provides an extremely powerful way to customize your Drupal website.
You can create a custom block via the block management screen (click on administer » blocks). Select the new tab and complete the form.
Each custom block has a title, a description, and a body. The content within the body can be as long as you wish.
Note: In Drupal 4.6, the title of the block is used as its identifier in the database table. Therefore you can only have one custom block without a title (this issue has been fixed in newer Drupal versions).
One workaround for this is to enter a title such as this: <!--title of custom block-->. This satisfies Drupal's need for a block title but the comment markers (<!-- ... -->) prevent it from being displayed.