A U value is a measure of how effective a material or a collection of materials is an insulator for example the combined components that make the outside wall. It is expressed in W/m2k, and shows the amount of heat lost in watts (W) per square metre of material (for example wall, roof, floor etc.) when the temperature (k) outside is at least one degree lower. The lower the u value, the better the insulation provided by the material.
So when it is cold outside and warm inside, U-value is the measurement of how quickly the heat from the inside leaks to the outside.
The build up of component materials in each element will dictate the overall U value. There are minimum u values set by the government which have to be met for each element of a building. These can vary if it is an extension being construction when compared to a new dwelling.