What exactly is Jitter?

http://en.wikipedia.org/wiki/Jitter

you can consider it as noise in time domain

yes dear
jitter is basically a variation in delay, of a packet,frame or a data chunk, in arriving their destination. This is a bad thing in the perspective of a system performance, because it makes the data delay and ultimately the system performance unpredictable

phase noise in time domain more prescisely, you can find formulars to convert phase noise to jitter

Jitter is a variation of excess phase with time.
It can be calculated by converting the RMS phase error to VCO periods.

The book, "PLL Performance, Simulation, and Design 4th Edition" Dean Banerjee
has intorduced about it.

Jitter is a short-time instability of the signal frequency, phase.
That can be see on the signal spectrogram, when around the sides of primary spectral garmonic is located a so called 'spectral pedestal'.
Range of jitter differs according to the level of this 'pedestal'.

The edge difference between ideal and actual waveform in time domain.
You can get more information by google.