Blocks are items which may be added to the left or right or centre column of any page in Moodle. They may also be added to the centre of the Dashboard. See the screencast: What are blocks?
Any block can be made 'sticky' so that it appears in all the contexts below, such as throughout a course or a particular activity - see Block settings.
Blocks can be placed on the side of the screen (if your theme supports it) via the Dock icons ( and ). If a responsive theme (such as 'Clean') is used, blocks will be accessible from the bottom of smaller screens, below the main content.
Note it is also possible to dock blocks when using the Clean theme if desired. You can theme the position of blocks in the centre of pages as well if you are able to create custom themes.
Available blocks include:
There are also many contributed blocks available for download from the plugins database.