Taking screen shot in Mac is easy, you don't need a special tool or software to do it. All you need to know is some keyboard commands to take a beautiful screenshots in PNG format.
List of Commands
Screenshot of Whole Screen
The following will take the screenshot of the complete screen. If you sound is own you will hear a camera click sound indicating the screenshot is captured and stored.The screenshot will be saved as PNG image file on the user Desktop.
Command + Shift + 3<!>
Screenshot of a Portion of Screen
The following will take the screen of portion of the screen that you choose.
Command + Shift + 4
Once you press the above key combination the shape of the cursor changes to a cross hair, and it display the screen co-ordinates in X and Y just next to it. Click and drag your mouse to do rectangular selection of the screen area that you want to capture. Just leave the mouse and it will take the screenshot of selected area. In this case also the screenshot will be saved as PNG image file on the user Desktop.