Three basic forms of noise in which it appears in electronic systems are:
1) Intrinsic noise: originating within the devices that constitute a circuit. (e.g., thermally generated Johnson noise and shot noise)
2) Transmitted noise: received with the original signal and indistinguishable from it.
3) Interference noise: picked up from outside the circuit.
This may be due to natural disturbances (e.g., lightening) or it may be coupled in from other electrical apparatus in the system or its vicinity.
(e.g., man-made noise from computers, switching power supplies, SCR controlled heaters, radio transmitters, switch contacts, etc.)