So the effective time of MIDI events varies within achievable latency bounds, but the precision is no larger than Resolution . This is the reason why MME mode MIDI timing can be as bad as latency. But not necessary. So average timing precision is around 15ms in this case. Pitch is measured in semitones and can be thought of as a MIDI key number.
Frequency is a physical characteristic of sound, exactly number of oscillations per second.
It is measured in Hz. E.g. if the signal oscillates 400 times per second its frequency is 400 Hz. The pitch and frequency are strictly related to each other. Going one octave up doubles the frequency. Goind one octave down halves the frequency.