A UHD or 4K display
is one with at least 8 million active pixels. For televisions, that resolution has standardized to 3,840 by 2,160. Digital cinema 4K (the resolution in 4K movie theaters) is slightly higher at 4,096 by 2,160. However you define it, it’s four times the number of pixels on a 1080p display, and over 23 times the resolution of standard definition television.
For starters, 4K HD is obviously much better than 1080p. In the space that a 1080p TV holds a pixel, a 4K TV of the same size can hold four. That makes for a significant jump in clarity, assuming you have native 4K source material to watch in that resolution.
Read more about 4k technology.