The temperature of a body is the measure of its molecular agitation. It's a function of the proportion of excited atoms and atoms in the ground state of a body. It represents the average energy of a molecule.

(We can understand this definition by defining an excited atom as an atom that vibrates much around its average position).

The amplitude of oscillations of an atom around its mean position increases with temperature. This is what causes the expansion, ie increasing the volume of a body when its temperature increases.

Unit: Kelvin
It is a basic unit of the International System because it is not related to the properties of a particular body.

Symbol K

Scale: The temperature difference between boiling water and melting ice in the atmospheric pressure is 100 K.

Zero Kelvin is the temperature corresponding to a zero molecular agitation. A body temperature that can not yield to any other heat. The Kelvin scale is 1 single fixed point which is conventionally the triple point of water at 273.16 K. (0.1 ° C)

Celsius scale:
In everyday life, and in diagrams, we use the degree Celsius which has its zero point at the temperature of melting ice and his 100 at the boiling temperature of pure water at a pressure of 1013.25 hPa.

It is no longer a basic unit since 1954, we should not use it.

Yet the express Thermometrists temperatures below 0 ° C in kelvins and higher temperatures in degrees Celsius.

The standard diagrams are graduated in degrees Celsius.

The graduation of the two units is the same difference expressed in K or ° C give the same number. (not coincidentally, the Cesius is something of an "ancestor" of Kelvin.)
Some calculations on the machines relate to differences in energy and temperature. On this condition we can use degrees Celsius.

Zero Celsius corresponds to 273.15 K.
Zero Kelvin is - 273.15 ° C
Problem: What is double 10 ° C?
Well no, not 20 ° C! Must be converted into Kelvin.
10C = 283.15 K multiplied by 2 = 566.3 K.
and to be more telling is 566.3 - 273.15 = 293.15 ° C.

Note: Do not use degrees Celsius to calculate the Carnot factor.