Boltzmann's constant (also referred to as the Boltzmann constant), written as kB, is named after the physicist Ludwig Boltzmann. For a biography on Ludwig Boltzmann please see here. This constant should not be confused with σ, the Stefan-Boltzmann constant.
The accepted value of the constant is:
The units of energy divided by temperature give a conversion that allows us to talk about thermal energy in terms of temperature. In other words, a particular temperature corresponds to a particular energy for a small particle (see for example thermal neutrons).
Boltzmann's constant is closely related to the ideal gas constant in that both are useful for the ideal gas law in determining the relationship among pressure, volume, and the number of molecules of gas and the temperature. The difference is that the Boltzmann constant counts individual molecules rather than the number of moles.
To learn more about Boltzmann's constant please see hyperphysics.