Understanding the basic principles of polar patterns will drastically improve any recording project you're working on.
A polar pattern is simply the sensitivity area in which the microphone registers sound waves.
The diagrams below show where individual polar patterns are sensitive to sound.
0° represents the front of the microphone, with 180° representing the back.
The rings are the decibel level at which the mic will register sound. The outer being 0db and most inner being -20db.
The cardioid polar pattern is an all use pattern. Microphones that use this pattern can be used in almost any scenario, from loud to soft environments, and with most instruments.
The Omnidirectional polar pattern has the same sensitivity to sound in every direction. This is a great pattern to use for field recordings as it pick ups very high frequencies and is the least sensitive to handling and wind noise.
The Wide Cardioid polar pattern is a mix between a Cardioid and an Omnidirectional pattern. This is great for recording groups, like a string quartet.
The supercardioid polar pattern is very useful in loud environments where sound bleed is a major concern, it also allows for high gain before feedback. This pattern is more directive than a cardioid pattern and has more side and less rear rejection.
The Figure-8 polar pattern has the most sensitivity at the front and back, and is least sensitive at the sides. It is great for rejecting any sound bleed that might come from the side while recording. It is the most sensitive to wind and handling noise.