What is the difference between low and high impedance microphones ?

Electret microphones which are high impedance and require a drive voltage to operate. Electret microphones are very sensitive and often contain a noise-cancelling circuit to reduce background noise. Standard GA headsets use electret microphones

Dynamic microphones are low impedance. They are not sensitive and usually only pick up noise within a few inches of the microphone so they tend not to pick up wind or engine noises and are ideal for noisy or open cockpit aircraft. Military headsets use dynamic microphones but the intercom or radio needs to have a special amplifier to be able to use them.

Dynamic microphones can be used on standard GA systems when a special amplifier is used.