On Windows Phone, the updates are divided up into two types: firmware and OS. Microsoft handles the OS updates, like Update 3, while OEMs use firmware to manipulate custom settings to work with their hardware.
Lumia Black is a forthcoming firmware update from Nokia for all Lumia devices. It was previously known as 'Bittersweet shimmer' during its development.
Lumia Black is set for release in late 2013 and early 2014 and it will premier on the Lumia 1520. While an official changelog is not known, there are numerous features that have been confirmed, including: