Thermal noise is created by the random motion of charge carriers due to thermal excitement. It was first observed and measured by J.B. Johnson in 1928 while working at Bell Labs and explained by H. Nyquist using thermodynamics and statistical mechanics. It is thus sometimes referred to a Johnson or Nyquist noise, depending upon if you an experimental physicist or a theoretician. Thermal noise is white noise. Every device, including resistors create noise. It is proportional to the absolute temperature.
Sv(0) = 4kBTR
Sv(0) is the spectral density that has no frequency dependence. T is temperature in Kelvin, R is the resistance and kB is the Boltzmann factor (1.38 x 10-23 joules/K).