Three types of noise are commonly observed in typical devices:
Shot noise. Electric currents are not continuous but are ultimately made up from large numbers of moving charge carriers, typically electrons. Shot noise arises from statistical fluctuations in the flow of charge carriers: if a single bit of data is represented by 10,000 electrons, the magnitude of the fluctuations will typically be about 1%. When looked at as a waveform over time, shot noise has a flat frequency spectrum.
Thermal (Johnson) noise. Even though an electric current may have a definite overall direction, the individual charge carriers within it will exhibit random motions. In a material at nonzero temperature, the energy of these motions and thus the intensity of the thermal noise they produce is essentially proportional to temperature. (At very low temperatures, quantum mechanical fluctuations still yield random motion in most materials.) Like shot noise, thermal noise has a flat frequency spectrum.
Flicker (1/f) noise. Almost all electronic devices also exhibit a third kind of noise, whose main characteristic is that its spectrum is not flat, but instead goes roughly like 1/f over a wide range of frequencies. Such a spectrum implies the presence of large low-frequency fluctuations, and indeed fluctuations are often seen over timescales of many minutes or even hours. Unlike the types of noise described above, this kind of noise can be affected by details of the construction of individual devices. Although seen since the 1920s its origins remain somewhat mysterious (see the next note).