Thermal noise, at -174 dBm/hz, is half phase noise, half amplitude noise. In the time domain it is a random noise vector of changing amplitude and phase. If you think of this small noise vector at the end of a bigger signal vector, you can see that the noise vector makes the composite signal bigger and smaller in length (AM noise), and appear to jitter back and forth in phase (PM).
Phase noise is a little unique, though, in that the electronic components can easily impart phase jitter to a signal that will be far in excess of that predicted by -174 dBm/Hz + noise figure. For example, say you have a microwave combline rod filter in the receiver, and it is physically placed close to a cooling fan. As the fan vibrates the chassis, the rods in the filter move around causing minute phase shifts. These minute phase shifts look like phase noise. Phase noise is also present in any sort of regenerative circuit, like a feedback oscillator, where low level noise is fedback on itself many times--resulting in significant phase noise. 1/f noise in an oscillator is an example of this. Finally, a very common form of phase noise is a microwave oscillator that is phase locked to a lower frequency reference--the microwave oscillators phase noise will be around 20 Log N worse than the reference's, where N is the microwave frequency/reference frequency.