Electret microphones (also known as “condenser” microphones) contain an electrically-charged diaphragm. They produce extremely little output and require an amplifier. Dynamic microphones use a moving coil (or similar) device which generates a voltage.
Dynamic microphones convert energy via electromagnetic induction, while Electret microphones do so with electrostatic principles.
Electret microphones are active, while dynamic microphones are usually passive.
Electret microphones are more complex than dynamic mics and are typically more costly.