This brief tutorial will show you how to change the default file format for screenshot (which is .png) to .jpg, .gif or even .pdf.
As we outlined in the introduction to Apple Keyboard shortcuts tutorial, taking a screenshot in OS X is as easy as 
+ 
+ 3 (Command+Shift+3, to take a screenshot of the entire screen) or + + 4 (Command+Shift+4, to take a screenshot of a region of the screen). The resulting screenshots will appear on your Desktop as PNG files.
Open up a Terminal by selecting Applications -> Utilities -> Terminal. Enter the following command:
defaults write com.apple.screencapture type PDF
Now sign out (click the “Apple Button” from the Menu Bar and select Log Out Your Name…. Remember to save all open documents etc before you log out.
When you sign back in again, take another screenshot. This time it will be a PDF.
To change formats again replace PDF in the line “defaults write com.apple.screencapture type PDF” to JPG, GIF or PNG to reset it back to the default.
One comment. After entering the command in terminal, you must press "return" for it to be active. Simply logging out and back in will not work. I know this may seem obvious to you, but the site IS for new Mac users and you cannot expect everyone to know that.
The first rule of technical writing is, "It must be written so it cannot be misunderstood." Otherwise, great job and I have recommended this site to many people making the switch. There's a business near me that's building Hackintoshes or converting customer's Windows systems if they have the right motherboards, etc. They like this site because it makes them look brilliant.