Last updated July 30, 2012.
Each Drupal project, either a module, theme, or installation profile, gets its own Git repository, hosted at http://git.drupalcode.org.
The Git repository for each project, regardless of project type, lives at the URL http://git.drupalcode.org/project/PROJECTNAME.git, where PROJECTNAME is the machine-readable name of the project.
Unlike CVS, Git does not currently offer the option of browsing all repositories in one place. Instead, you must access each repository directly by fully-qualified URI, e.g. for Drupal core, http://git.drupalcode.org/project/drupal.git . If you want a listing of repositories, you will need to consult the listings of projects on Drupal.org itself: modules, themes, and distributions.
Note: Browsing capability will be added in a future phase of the Git rollout.
For Drupal core, there is a Git branch for each major version of Drupal. For example, the last few branches of http://git.drupal.org/project/drupal.git are 5.x, 6.x, and 7.x.
Modules and themes use branches for each new major version, as well as releases for new versions of Drupal core. Their branch naming convention include the Drupal core version and the module's version number. Example branches include 6.x-1.x, 6.x-2.x, and 7.x-2.x. For more details about branch names, see Release naming conventions.
Tags are used for individual releases of a project. For example, there is a new tag for each new minor release of Drupal core, a few of the latest being 6.19, 6.20, 7.0-rc-4, or 7.0.
Modules and themes use a tag naming convention including the Drupal core version and the module's version number. Example tags include 6.x-1.0, 6.x-1.1, and 7.x-2.0-alpha. For more details about tag names, see Release naming conventions.