Drush is a command line shell interface for Drupal which can be very helpful for all sort of automated tasks like backups, package management, updates etc..
Installation
The preferred way to install Drush is via the official PEAR channel but it can also be installed manually (which might be the only way if you are on a shared server).
Backup/restore
archive-dump is useful to backup code, files and database into a single file while archive-restore can expand that single file back into a Drupal site.
Package management
Drush can download (pm-download), enable (pm-enable), disable (pm-disable) etc modules and/or themes.
Updates
Something I’ve always envied to WordPress was the possibility to update its core with a simple click.
Drush can do exactly the same, updating the core with one click single “enter” thanks to pm-update.
The same command can update modules and themes as well.
This was just a simple introduction to Drush in case you didn’t know it already.
Have a look at the official documentation to find out all Drush can do for you.