The Lumia Black update includes the latest Windows Phone 8 GDR3 build, bringing lots of new features, like the ability to create folders on the start screen, Nokia Glance 2.0, enhanced notifications, Bluetooth 4.0 LE support, and so on. The update brought camera improvements to the newer Lumia 1020 and 925 earlier this month, so the camera capabilities of the 920 and 820 might also be enhanced.
As always, you can manually check for updates on your WP handset by going to Settings -> Phone update -> Check for update.
In case you didn’t know, AT&T is currently offering the Lumia 920 for free on contract, or for $299.99 without any agreement. The Lumia 820 is no longer available to purchase - at least not from AT&T’s website.
source: Nokia Care US via WPCentral