Sometimes words alone aren’t enough to describe something, and that holds true with the iPhone and iPod touch, too. The iPhone 3G, and iPhone or iPod touch with Software version 2.0, make it easier to speak with images thanks to the new screenshot feature, no extra applications needed.
A screenshot is a digital snapshot of what your display is showing. In the case of the iPhone and iPod touch, that can include what is visible on the Home screen, or in any application on the handheld device.
To take a screenshot on your iPhone or iPod touch, quickly press and release the Home button and power button at the same time. Your handheld’s display will flash white briefly to let you know a screenshot image was captured. 
Home + Power = Screenshot
You can find screenshot images you take safely tucked away in the Photos application. Since you can email images from the Photo application, those pictures aren’t trapped on your iPhone or iPod touch. Just email the shots to anyone you want to share them with.
If emailing an image isn’t what you want to do, just connect the handheld to your computer and let iPhoto on the Mac, or Camera Wizard on Windows, copy it off.
Screenshots are handy if you want to show someone a picture of how something looks on your screen, and also are a handy way to show friends your high scores in iPhone and iPod touch video games.