A ring oscillator is just a kind of oscillator based on an odd number
of inversions and propagation delay around a loop. It can be very fast,
but is not very precice (frequncy depends heavyly on temperature, supply
voltage, etc). One application of ring oscillators is to determine inverter time delays as a means to evaluate the CMOS fabrication process
A phase locked loop is a system that (in its simplest form) includes some sort of
electronically variable oscillator and a phase detector that compares
that oscillator to an external signal. The output of the phase
comparator is linked to the oscillator in such a way that it drives
the oscillator into synchronism with the external signal.
A PLL has a VCO (voltage controlled oscillator) a phase detector (measure
the phase relation of two signals) and a loop filter. Often there are
additional dividers. A Pll can multiply clock signals, it can remove a delay from clock signals and it can clean up clock signals (remove jitter).