Microphone Polar Patterns
The microphone's sensitivity to a sound relative to the direction or angle of arrival at the microphone is known as its polar pattern or directionality.
There are a many different patterns designed into microphones. The three basic patterns are omnidirectional, unidirectional, and bidirectional.

The Unidirectional patterns are most sensitive to sound arriving from one particular direction, and less in all other directions.
The most common type is the cardioid (heart-shaped). This mic pattern is used if a more isolated pick-up is desired.
cardiod cardiod
Unidirectional mics also come in other variations like the supercardioid and hypercardioid. These have a very focused pattern and are used when the source is far away or there is a lot of ambient noise. The hypercardioid picks up less from the side, but picks up more from the rear than the supercardioid.
supercardioid supercardioid

AND   Hypercardioid   hypercardioid
cardioidhypercard.gif From the polar patterns superimposed on the mics above, you can see that a cardioid mic provides most rejection of sounds arriving from the rear, while hypercardioid (and supercardioid) mics have two rejection nulls off axis (at appx 120degrees).

The Bidirectional pattern has a full response at both 0 degrees (front) and 180 degrees (back). It is sometimes used to capture two sound sources and in certain stereo techniques. It is generally side-addressed, versus end-addressed.
bidirectional bidirectional

The omni pattern has a equal response at all angles. Its pickup angle is a full 360 degrees. These mics have a very natural sound and are good if the room’s ambiance is desirable. They can be side-addressed or end-addressed