There is no "Aegir" project on drupal.org, but the Hostmaster profile is a component of the Aegir hosting system.
Hint: to install aegir for the first time, you should be using the provision drush command which has a neat script that will do everything for you.
Ægir is a new set of contributed modules for Drupal that aims to solve the problem of managing a large number of Drupal sites. It does this by providing you with a simple Drupal based hosting front end for your entire network of sites. To deploy a new site you simply have to create a new Site node. To backup or upgrade sites, you simply manage your site nodes as you would any other node.
The hostmaster profile ties together the functionality of the hosting front end and provisioning back end into a complete package. It handles initialization and auto-detection of your initial configuration settings, and directs you to the friendly installation wizard. To start using the Aegir system, you will install a new site using the hostmaster profile, which you will then use to manage all of the sites you host.
For more information on the system, and the other components, please take a look at the Aegir overview wiki page
To find out how to install Aegir rather than download the tarballs here, read the documentation.
There are several branches in version control, here's an explanation of what the different branches are for:
- 6.x-1.x: The current, stable version of Aegir, only bug fixes (and essential features) will be committed to this branch.
- 6.x-2.x: The upcomming version of Aegir, using Drush 5, frontend in Drupal 6
- 7.x-3.x: The current development version of Aegir (Drupal 7 frontend), roll patches against this branch for both new features and bugs. See #1261030: Roadmap: D7 port of Aegir