Mono Tones were the original ringtones that played a tune. They sounded great at the time when compared to "bleep bleep", but have now been left far behind by modern phones.
Polyphonic Ring Tones were the first ring-tones to offer a fairly realistic musical sound. Based upon the MIDI (Musical Instrument Digital Interface) format, phones that play Polyphonic Ring Tones, synthasise the original music.
RealTones, play a digital recording of the original sound. Hence the name. They curently offer the most realistic sound.