If you are comfortable with PHP and would like to participate in the testing portion of our development cycle and report bugs you find, beta releases might be for you.
By their nature these releases are unstable and should not be used any place where your data is important. Please backup your database before upgrading to a beta release. To hear about the latest beta releases your best bet is to watch the development blog and the beta forum.
Check out our Beta Testing Handbook for more information on helping make the next release of WordPress great!