Coming first for the Lumia 1020 and Lumia 925, the update will eventually follow for other devices “in the coming weeks”. If you want to check if the update is available for you now, you can do so by going to Settings -> Phone update.
In among the improvements are several improvements to camera functionality like Nokia Refocus and Nokia Camera, as well as Nokia Beamer for sending documents, pictures or other content to other screens by scanning a QR code. It also includes App Folder, to allow users to personalize the Start screen by grouping similar apps and settings into one folder. It also brings Nokia Storyteller and Nokia Glance Screen to more devices.
Nokia Lumia Black Update Begins Rollout [Nokia Conversations Blog]