Electret microphones are very similar to condenser microphones but instead of needing a power supply to give a polarizing voltage to the back plate they are manufactured with a permanent charge, albeit of a lower voltage. Power is still needed however for the pre-amplifier, phantom power is usually used for this in a stand alone microphone; when the electret is part of another piece of equipment e.g. a stereo microphone in a camcorder then the power is supplied from the built-in power supply.
This means you can get a smaller microphone with similar fidelity to a condenser microphone. The lower voltage on the plate means it doesn't have quite the same wide dynamic range or low self-noise characteristics of a condenser microphone.
Electret Lapel / Lavalier Mics
As lapel microphones their pickup pattern is usually Omni (some cardioid versions are made) but check that this is the case to avoid a muffled sound - some recordists use them upside down while indoors to avoid wind blasts (plosives) from the person's breath in order to keep their appearance less intrusive by not having to use a windshield.
Outdoors it is essential to use windshields or to conceal them under clothing that shields them from the wind but is not of such thickness that the audio is muffled. Sony microphones produce probably the best known range of lavaliers - the ECM range, their size has progressively decreased and their fidelity increased over the years. Other well known manufacturers include Sanken, Beyer, AKG and DPA.
The cost of producing electret microphones is much less than that of condensers and this, along with their size, explains their wide use as lapel (clip, lavalier) microphones and in camcorder stereo microphones and headset microphones.
In addition their immunity from electro-magnetic interference makes them ideal for use as USB and computer microphones. Some microphone manufacturers such as Audio Technica and Bruel and Kjaer custom build them into some of their high end microphones instead of using condenser technology.
Lavalier electret microphones can be very versatile in their placement since the capsule is physically small and connected to the pre-amp by several meters of thin pliable cable. They can be put inside acoustic guitars or on saxophone bells where they are always in the same position and so don't go 'off - mic' as the musician moves about and for this reason they are very popular in theatre and stage musicals for headset microphones with a wireless microphone system.
As with condenser microphones care must be taken to avoid moisture and stage makeup getting into the microphone.
The lavalie's thin cable makes it delicate so care must be taken in avoiding damaging the cable or excess pulling force between the cable and the capsule.
Physical noise on the cable can be more readily transmitted to the capsule itself as well, so avoid tensioning the cable.
Electret Microphones - Summary
The electret microphone is cheap to produce and has similar characteristics to the condenser microphone. It can be made to very small dimensions and doesn't need a power supply, though its pre-amplifier will need power, albeit of a low voltage and current.
Strictly speaking Lavaliers aren't produced anymore, they consisted of a necklace with a holder for an, at the time, small dynamic microphone (about two inches long), it allowed the wearer to have both hands free, nowadays it more commonly refers to a 'tie clip', 'lapel' or 'personal microphone' worn somewhere on the body, electrets are mostly used in this role because they are so small. The origin of 'Lavalier' comes from Madame Lavalier who was well known for the large ruby necklace she wore.