CRTs and plasmas can get burn in, which is more accurately called uneven phosphor wear. Since they are phosphor based technologies, they can permanently have certain areas that are worn out faster than others and visibily show this issue.
LCD, which is what you most likely have, has image retention. In LCD land, this occurs because an LCD panel (always 3 with LCD projection) retains an electrical charge in certain LCD pixels (dots) after a static image has been left on for a long period of time. So, you see an effect very similar to burn-in. Most often though, simply turning off the display for 24 hours or so will allow the eleticity to disapate from the panels and the image retention will completely disappear.
DLP does not suffer from this at all from my (limited?) understanding.
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