Microphones are designed to read and interpret the sound around them. This is really their only function, and if they cannot do that well then they are completely worthless. This is why microphones with specific abilities have been developed so that they can respond better to situations that they are designed for. The Cardioid and Omni microphones both represent a completely different way to pick up sound, which makes each important for unique situations.
What defines the Cardioid and Omni microphones are their polar patterns, which are also called pickup patterns. These polar patters refer to the pattern that the microphone receives the frequency. The specific pickup pattern refers to what direction the microphone picks up this pattern, such as direct on from the tip or off to the side. Along with this is a proximity effect that mimics the distance of the sound. Instead of recording all sounds with equal intensity the proximity effect of the microphone differentiates between sounds that were recorded close up and those recorded from far away.
The Omni directional microphone has what is considered a full range pickup pattern. It can pick up sound in a three hundred and sixty degree range around the microphone. This means that it will get sound from all angles without preferences toward any direction. Because of this polar pattern the Omni microphone also has a relatively small proximity effect, which means it is harder to differentiate which sounds were far away.
The Cardioid microphone has a directional pickup pattern, which also gives it the name of unidirectional. This pickup pattern is defined as a ninety-degree angle coming from its tip. This means strait on from the microphone the polar pattern goes forty-five degrees in one direction and forty-five degrees in the opposite direction. This Cardioid polar pattern is good when you only want to get one specific sound from a specific source and want to avoid the other sound sources in the area. Because of this design the proximity effect is much stronger on the Cardioid microphone. The Cardioid microphone also has subdivisions with the Hyper Cardioid and others with similar names. These often refer to the shape, strength, and ability of the Cardioid microphone. They may change and vary somewhat, but they remain the unidirectional pickup pattern
Other Pickup Patterns
Bothe the Omni and the Cardioid microphones will get audio that comes from the front, though the Omni will get also around the sides. This puts both of these at odds with bi-directional microphones that have a one hundred and eighty degree pickup pattern. This is often the case with audio recording microphones that are used in audio engineering. These are often good for instruments, but not nearly as great when associated with video production.