As Delluser1 said, Noise (ripple) is what you see as variations on the voltage Line. Need an O'scope to measure, computer with software can not do this. Time interval to measure generally in millisecond to nanoseconds. Sample rates of software is way to slow
Primarily 4 catigories:
Line noise - generally very low and is at line frequency (60 Hz for US).
Switcher frequency - this is 20 -> 40 Khz, also fundamentals of this (ie 2x, 4 times frquency.
Load variations - caused by rapid flucuations of the load. ie when you switch from 2 D to 3D graphics.
Misc frequence High Freq caused by poor shielding and or poor filtering. An example of this would be seeing your Clock freq as ripple on on +12 V rail.