Full description through the link, but here's a taster.
Among the features available in the Steam Overlay on PlayStation®3:
* • Play cross-platform from your PlayStation®3 with Steam friends on Windows or Mac.
* • Access your Steam friends list, enabling you to chat with, add, or remove Steam friends.
* • Access redeemed downloadable content registered in Windows or Mac, and vice-versa
* • View your Steam friends’ community profiles.
* • View your earned Steam Achievements, as well as global achievement stats, for Portal 2.
* • See the latest news and announcements for Portal 2.