This is an excerpt from a course on the excellent Lynda training site. It's about microphone directional response. I wrote and article about this here - Directional and Omnidirectional Microphones – What Are They Good For? .
This video (below) is a great additional piece to help you better understand the same topic. Understanding how a microphone pics up sound is pretty important in both recording and live sound.
You can achieve a lot of cool stuff, like lowered noise (with a cardioid's "null" pointing to the noise source), by knowing the polar pattern of your microphones.
Check out the video on microphone directional response below: