It's called Image retention and has been discussed many many many times here.
I don't know where this guy has been. There has been no other issue discussed in more places than this. But hey...That's what this forum is for. Nobody should be shy about asking their questions or voicing there concerns. This is a safe place, for experts, novices and newbies alike. And besides, it would have been difficult for him to do a search considering he didn't know the name of the issue. Easier to just ask away.