Omnidirectional

Uses: Capturing ambient noise; Situations where sound is coming from many directions; Situations where the mic position must remain fixed while the sound source is moving. Also used as testing and measurement mics.

Notes: Omni's are pretty weird mics in that they pick up everything. For special affects they can be very cool, but typically if you use an omni you will find that the sound you want is being covered up with noise and sounds you don't want.

Cardioid

Cardioid means "heart-shaped", which is the type of pick-up pattern these mics use. Sound is picked up mostly from the front, but to a lesser extent the sides as well.

Uses: Applications where you want to pick up sound directly in front of the mic and at the same time allow the source to move without losing the signal. These are the most common microphones used in live sound. They are great for vocalists and amplified instruments.