Mac OS comes with in built short cut keys similar to the Windows Print Screen button system.
Its a bit harder to use but a lot more flexible.

There are 3 different types of screenshots you can take using the shortcuts.

1. Capture the entire desktop

Apple ( Command ) Key + Shift + 3

2. Use your mouse to select a region of your desktop

Apple ( Command ) Key + Shift + 4

Follow this key combination by selecting the region you wish to capture with your mouse

3. Capture a specific window

Apple ( Command ) Key + Shift + 4 followed by Spacebar then use your mouse to select the window to capture

 

All these commands save the screenshots do your desktop with the current date and time
eg Screen shot 2010-03-15 at 5.20.33 AM.png