If you are a user or integrator that installs CiviCRM by downloading the zip or tar ball from the CiviCRM site, this transition won't affect you in any way.
If you are a developer, or an integrator that currently uses the CiviCRM subversion repository to manage your sites or as part of your deployment system, please read on.
The main difference between subversion and git is that subversion is a centralized revision control system whereas git uses a distributed model. By changing to git, CiviCRM will be able to more easily organize a large group of developers to contribute code to the project, while still maintaining a small team of core developers who have write access to the officially distributed CiviCRM code base.
No dates have been set for the transition, but now is a good time to get familiar with git and start to think about what this transition might mean for your development and integration process.
The git documentation page is an excellent place to start if you are looking for documentation or tutorials. It also includes a page specifically for helping people transition from subversion.
Some ideas that were suggested for a smooth transition include:
We're still considering git hosting options, but Gitorious seems to be the front runner at the moment (we are not considering github.com because it runs on proprietary software).