There are a few different pick up patterns for microphones. This is a 3dimensional way of describing from which direction they are deisgned to pick sound up from. The following diagrams are called polar patterns.

Omnidirectional

Some microphones you will come accross will be omnidirectional. It is the pattern typically found on cheaper microphones. It could come in handy when using a mic as an atmosphere mic or for general pickup.

Cardioid

This pattern is the most commonly used. What the picture is describing is where the microphone will be the most sensitive. It's pattern is used mostly for vocal work. The lack of pickup at the bottom of the mic helps to reduce noise made my holding the mic.

Hypercardioid

Hypercardioid is an exaduation of Cardioid. It's simply more concentrated. Here's the diagram..

Bidirectional

Bidirectinal microphones do what they say on the tin. Two directions. It's not a common case as it's not that practial apart from maybe an interview between two people with the mic in the middle.

(I hope you can see the 3Dness here :) )