Basically there are to ways of doing screen shots on Mac X:
1. Basic
2. Using Grab program
Basic
To capture entire screen you press Command-Shift-3 and the Picture 1.png image file is created on your desktop. You can also use Command-Shift-4 to select only part of your screen, and just like before Picture.png file will be created. On the other hand using Command-Shift-Control-4 instead of file, the screen is copied to the clipboard.
Using Grab program
This program is in Applications -> Utilities folder.
I personally prefer the Basic way of doing screen shots. For me it is more convenient. Moreover I prefer png files over tiff files, which are created by Grab.
