Your telling your friend about how you have your apps arranged on the screen, or you want to explain a screen in an application your in, or maybe there is a Web site you want to share a image of.
When the occasion arrises that you need a screenshot;
Hold down the main front button (called the ‘Home’ button)
Then press and release the button on the top of the iPhone
The screenshot is saved without changing anything your doing so you can continue on. Just go to your Photos area to view the screenshot. From there you can email it or retrieve from iPhoto the next time you sync to your desktop.
The resulting image is a png file that opens to a image 320 x 480 pixels, 72 dpi, and 16.7 million colors.
Click on image to enlarge –