Dynamic drivers = a driver where a coil -that carries the audio signal- moves in a magnetic field -due to the alternating current of the audio signal- a diaphram. In a 'traditional' cone spraker the diaphram is usually conically shaped and made of paper, metal, ceramic, plastic or some combination of these materials.
Plannar drivers are large sheets of usually mylar that have a conductive surface which carries the audio signal, and is placed betwwen two field arrays of magnets - the alternating current moved the membrane back and forth creating sound very similar to the dynamic driver above.
Electrostatics: a membrane with a conductive coating/layer that carried the audio signal mounted in between two High Voltage screenes one positivly charged, the other negativly charged and the AC of the audio signal causes the membrane to move etc... The voltage on the screen varries by design and manufacturer, the old AcuStats were in the 5Kv range, Beveridge 2 and 3 were in the 1.5Kv range.
As has been stated above, Horns usually refer to an enclosure. Some of the most common are, Altec Voice of the Theatre, Klipsch (old) and Avant Garde (new). There are drivers called 'Compression drivers' that were designed to be used in a Horn enclosure; however they are just dynamic drivers designed to work in a Horn enclosure.
As to the efficiency, Horns are the highest Klipsch and Altec VoT can be run to LOUD levels by a walkman/discman, Electrostatics are at the other end, with a max SPL of some in the 88dB range.
Sound - well that is why they make vanilla. Some have staunch adherents that decry anything else, and then there are people who have speaker systems that have elements of all three.
The inefficient units tend to be harder on amps, and some can have wide impedance ranges.
Plasma speakers are a different breed all together. Plasma speakers are designed to displace air to make sound by generating a gass stream (usually from a point source - read omnidirectional radiation pattern) into the listening environment. They are not widely manufactured or sold, and there are some hair-raising stories told about some of the more infamous units - some thing about ozone build up and a fire, but I digress.
As has been stated above, Horn enclosures are usually designed to take advantage of the strength of, or to correct a weekness in a driver design. The use of Voit Pipes and folded horns arround full-range drivers is one example.
Having said all of this, for my system, I chose spealers that could be driven well by my amp and revealed more that others but were within my range ($$). I have lived with ESLs, Beveridges (2SW), Maggies, Altec, Klipschs and have settled on some old ProAcs, Shahanians, Celestions and JM labs, all Dynamic units in the end.
Happy listening