Temperature Scales

The Celsius (or centigrade) scale:

Temperature is the average kinetic energy of the particles of a substance. 
Temperature scales have been devised around common, arbitrary reference points as to the state of common substances. 
Water is one of these common substances. 
For example, the freezing point and the boiling point of water respectively were chosen to indicate zero degrees (00C) and one hundred degrees Celsius (1000C -- also known as centigrade). 
Normal average human body temperature was then standardized at 370C, and room temperature is taken to be 200C.
Standard temperature and pressure (S.T.P.) will set the temperature at 250C.
The Kelvin (or absolute) scale:
Scientifically, it makes more sense to relate the temperature of a substance to motion of the molecules within the substance and the kinetic energy of these molecules. 
If the molecules were to stop moving completely then that would indicate an "ideal" absolute temperature of ZERO. 
This point is known as absolute zero or ZERO KELVIN (after Lord Kelvin).
To arrive at this temperature we need to extrapolate and interpret the cooling curves of common substances -- since it is virtually impossible to reproduce these conditions in a laboratory.