Microphone directionality is the variation in sensitivity 360 degrees around the head of a microphone.
There are a variety of polar patterns found in microphone design including:
Omni Directional: Omni microphones pick up sounds equally in all directions. They are great for recording room ambiance and capturing group vocals.
On the downside, they lack background noise rejection and are prone to monitor feedback, which makes them unsuitable for loud and noisy venues.
Bi-Directional (Figure-Eight): Bi-directional microphones pick up sound in two directions; in front of and behind the microphone, with rejection from both sides.
The front and back sensitivity make them idea for stereo recording and for capturing two or more instruments.
Cardioid/Uni Directional: Cardioid microphones pick up sound from the front and offer maximum rejection at the rear. They are useful for reducing bleed and leakage between sound sources, making them ideal for live performances and situations where feedback suppression is needed.
Cardioid microphones are extremely common and can be found in most studio cupboards.
Hyper Cardioid Directional: Hyper cardioid microphones have a narrow pickup pattern, but also have some rear pickup.
Hyper cardioid mics have better isolation and higher resistance to feedback than cardioids. They are ideal for loud sound situations.
On the downside, as there is some bleed at the rear, you will have to position unwanted sounds on the dead spot sides.
Super-Cardioid Directional: Super cardioid microphones have a narrow pickup pattern like a hyper-cardioid, but with a tighter rear pickup.