The traditional figure-eight is the simplest directional microphone - its diaphragm is completely open to the air on both sides. This causes it to respond only to the instantaneous difference in pressure between the two sides of the diaphragm.
The shotgun or rifle microphone is an extreme example of the standard cardioid implementation. To achieve the extremely narrow field, the sound waves are conducted through an array of slots in a long tube - the interference tube - to the back of the diaphragm. This gives the shotgun microphone its familiar appearance.
The standard stage and studio microphones obtainable almost anywhere are cardioid - they have a sensitivity contour that is heart shaped with the point smoothed off.
The reflector microphone is typically a conventional omni mounted in a parabolic sound reflector or dish, usually made of plastic. Reflectors range from 18 to 38 inches ( 50 cm to 1 metre) in diameter.