Here are the release notes:
This beta version of iTunes introduces an elegant new design, the ability to share purchases with your family, and improvements that make organising your library fun and easy.
Streamlined Design. iTunes now has a beautiful new look and simplified design that makes browsing your library easier and more enjoyable.
Family Sharing. Share your purchases from iTunes, iBooks, and the App Store with up to six people in your family — without sharing account.
Improved Playlists. You can now see your full library and playlists side by side, making it even easier to add your favorite songs to playlists.
Redesigned Info Window. The Get Info window has been completely redesigned featuring a clean look that displays the most useful information about your media.
Developers can download and install iTunes 12 beta from the Updates tab in Mac App Store, or alternatively by clicking Software Update from the Apple menu.
We’ve taken a detailed look at the redesigned interface in this post with a lot of screenshots.
(Image via @73steven)