A block is a container of content that can be placed within a region. You could, for instance, create a text block, and insert it into the left sidebar of your website. You can add multiple blocks to a single region, such as multiple menu blocks added to a sidebar. Shown below is an ‘administration menu block’ as well as a ‘Who’s new’ block.
Warning: Before adding PHP programming code to a block, make sure your site is backed up. You should have a functioning backup/restore plan in place with your support provider. Also test any PHP in an article/story before saving it as a block.