Full High Definition (FHD) is the resolution 1920x1080P. Ultra High Definition (UHD) is the resolution 3840x2160P. What this means is that there are more pixels per inch (PPI) in a UHD TV. This gives a higher resolution on the TV. UHD is used on larger TVs so you can sit closer to the TV and still enjoy a crisp, clear picture. Below is a diagram of the different resolutions:

What Is The Difference Between FHD And UHD?
What Is The Difference Between FHD And UHD?