Head on this page.
image retention, which is an uneven aging of the phosphors in a display device, can occur on any display that uses phosphors to generate an image, including tube TVs, projection TVs that use CRTs, and plasma TVs.
Such uneven aging happens when bright, static images are left onscreen for an extended period of time, which can leave a permanent "shadow" that is always visible.
It often occurs because the contrast and brightness settings on the TV are too high. In addition, screen savers, and brightness level adjustments can dramatically reduce any chance of image retention. Use common sense when it comes to your plasma TV; don't pause video games or watch TV stations with station logos onscreen for long periods of time, and use one of the many display calibration DVDs available today for properly setting brightness and contrast.