To capture a screen image (often used for diagnostics, bug reporting, or explaining a question) do the following:
To take a screenshot, simultaneously press the home and sleep/wake buttons. Do not hold them. The screen will flash, you will hear the camera sound, and your iPhone's display will be captured and placed in the Camera Roll.
For information about devices (ipods) without cameras go to this link on Appple's web site: http://support.apple.com/kb/TS3195