There are three main temperature scales used in the world - Celsius, Fahrenheit, and Kelvin. These are compared in the following table.
One can see from this that the size of 1 oC and 1 K is the same, while both differ from the size of 1 oF; in fact, the difference between the Celsius scale and the Kelvin scale is simply a shift by 273 o. The Kelvin scale is more convenient to measure substances with very low temperatures. Moreover 0 Kelvin has a very deep physical significance: absolute zero, as the name suggests is the lowest temperature that can, even in principle, be achieved in Nature. It is the temperature associated with empty space that is completely devoid of all all motion and/or energy. In practice it is impossible to obtain, although one can get arbitrarily close. As we will see in the chapter on cosmology, not even the empty space between distance galaxies is at absolute zero: it contains energy and an associated temperature of about 2.7 Kelvin.