The most common are 6dB, 12dB and 18dB slopes. A 6dB slope will reduce the power at a rate of 6dB per octave starting at the chosen crossover frequency, creating a gradual slope of power reduction. The 12dB and 18dB per octave slopes work the same way except that a 12dB slope has twice as much reduction per octave as the 6dB slope and an 18dB slope has 3 times the amount of reduction, thus cutting off those frequencies 3 times as fast. Iron core coils are best for lower frequencies. Polypropylene are the best capacitors.
Mylar capacitors are next best.
Non-polarized electrolytic are the most commonly used. This stands for Sound Pressure Level measured at 1 watt of power and at a distance of 1 meter from the speaker. From what I have seen in speaker catalogs, I think most measurements have been copied from the spec's of the individual horn or midrange components instead of an actual reading from all components operating together in the speaker cabinet. The least expensive of the two is the Piezo driver. Most people think of a small round tweeter when you say Piezo, but there are also larger, threaded horn drivers that have a piezo element instead of a voice coil.