On an iPhone, iPod touch, iPad, or iPad 2, it's simple [that said, we are still asked about it]. You can take a screenshot simply by pressing the Power Button and the Home button at the same time. You'll hear the iDevice make a shutter sound if not muted, and you'll also see the screen blink, and that's all there is. You have a screenshot.
To find the screenshot, simply enter the Photo gallery on the device. You can then Share it in a number of ways, including emailing it.
This assumes you are running iOS 2.0 or higher. This ability did not exist in iOS 1.x.