Typical polar patterns of different types of microphone.
A polar pattern is a circular, 2-D plot that indicates the directional response of a transducer. While polar patterns are commonly used to show microphone pickup patterns, they can also indicate the dispersion of a loudspeaker.
Interpreting polar patterns, even with something as simple as a common cardioid microphone, can be fairly complex. For one, the polar response shown on paper doesn't indicate the fact that a microphone's pickup pattern is actually a 3-D space around the mic. Second, the polar response of any microphone is frequency-dependent. For example, a ultradirectional "shotgun" microphone is extremely directional at high frequencies and much less so at low frequencies.