Electronic noise is any fluctuation in voltages that you don't want.
It could be from the operation of a chip, capacitors help smooth this.
Here is one example of noise being generated and being suppressed.http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
Noise can also be generated by thermal agitation of electrons, this is know as thermal noise. There are other mechanisms that generate noise but in digital electronics this is normally too small to be a problem.
Noise can also be injected into a circuit through radiation when the circuit is acting unintentionally like a radio receiver.