The difference is subtle: A perpetual motion machine could in theory be constructed, and will continue to move as long as it's components don't wear out due to particle decay or some kind of impact. It simply means that a machine will continue to move.
The cleverest pseudo perpetual motion machines are very hard to debunk, some of the more intricate ones rob the earth of a little bit of momentum to function.
A 'real' perpetual motion machine (one without external energy input to overcome the inevitable engineering issues) has never been constructed but can't be entirely ruled out.
An 'over unity' machine produces more energy than goes in to it, and is an engineering impossibility.
For years I've had this up:
http://web.archive.org/web/20050113055344/http://www.greenbi...
But sadly no takers :)