Ultra HD is a moniker created by and agreed upon by the major display and CE manufacturers pertaining to the next evolution in the consumer HD broadcast and video standards. In October 2012, the Consumer Electronics Association introduced the term "Ultra High-Definition" or "Ultra HD" and specified some core characteristics. To earn the Ultra HD designation, a display device must: 1) have a resolution that consists of at least 3,840 horizontal pixels and 2,160 vertical pixels; 2) must have an aspect ratio of at least 16:9; and 3) must have at least one digital input that can accept a native 4K-format video signal at the full 3,840 x 2,160 resolution.
The Consumer Electronics Association chose the name Ultra HD over the more commercial term 4K, in part because they felt that the term Ultra HD was easier for consumers to grasp but also because Ultra HD encompasses more resolutions than 4K. The commonly accepted definition of 4K is a video resolution of at least 4,000 horizontal pixels, whereas Ultra HD includes the slightly lower Quad Full HD resolution of 3,840 horizontal pixels. Quad Full HD is so named because it is exactly four times the resolution of current 1080p sources (1,920 x 1,080 = 2,073,600; 3,840 x 2,160 = 8,294,400).
While the Ultra HD umbrella covers higher 4K resolutions, the vast majority of Ultra HD TVs have a Quad Full HD resolution. This trend is likely to continue because QFHD scales proportionately from a 1080p source (2 x 1,920 = 3,860, 2 x 1,080 = 2,160) and perfectly suits the 16:9 aspect ratio commonly used for broadcast HD content.
On its own, the higher resolution of Ultra HD may not present a dramatically visible improvement over 1080p at common TV screen sizes, viewed at common seating distances. However, as the Ultra HD format evolves, improvements in color and dynamic range can yield much more obvious improvements in picture quality. Many early Ultra HD TVs adhered to the same HD standards as 1080p TVs--namely, an 8-bit color bit depth and the Rec 709 color space.
In 2013, the International Telecommunication Union, which sets the standards upon which the production, broadcast, and display industries rely to ensure that what's being done on the content-creation end is being accurately transmitted and displayed on the TV/projector end, released the BT.2020 or Rec 2020 standard for 4K Ultra HD content, which includes a much wider color gamut and higher 10- or 12-bit color bit depth. As of 2015, manufacturers are beginning to incorporate 10-bit panels and wider color gamuts into their Ultra HD TVs to move closer to this standard, although full Rec 2020 color hasn't been achieved yet.
As of 2015, manufacturers are also adding High Dynamic Range capability to improve the overall brightness and dynamic range of Ultra HD TVs, which is another important piece of the overall puzzle. You can read more about it here.