In addition to classifying microphones by their generating elements, they can also be identified by their directional properties, that is, how well they pick up sound from various directions.
Omnidirectional microphones are the simplest to design, build and understand.
They also serve as a reference against which each of the others may be compared. Omnidirectional Omnidirectional microphones pick up sound from just about every direction equally. They'll work about as well pointed away from the subject as pointed toward it, if the distances are equal. The physical size of the omnidirectional microphone has a direct bearing on how well the microphone maintains its omnidirectional characteristics at very high frequencies. The body of the microphone simply blocks the shorter high-frequency wavelengths that arrive from the rear.