Ultra HD or "ultra high definition" is the term used to refer to screens with an image resolution of 3,840 pixels across and 2,160 pixels on the vertical axis, in total the screen contains a total of approximately 8,294,400 pixels. 8 megapixels which is four times the resolution of a high definition or HD screen that is 1080 pixels wide.
For this reason Ultra HD is often referred to as 4K as it is four times the resolution of a high definition screen.
Yes, 4K and UHD are used interchangeably and refer to the same screen resolution of 3,840 x 2,160 pixels on consumer televisions and displays.
The large number of pixels on a 4K UHD screen allows us to see images with a high level of detail and quality.
Such amount of pixels on a screen is achieved by reducing the size of each pixel (a pixel is a color square) which is good since we will not be able to distinguish space between each pixel and we will be able to see a much more real image than on an HD screen.
The higher number of pixels also allows much larger screens to be made without losing quality.