The main difference is a few hundred dollars!
Essentially, a GPS just shows you your position as a longitude / latitude reading. (i.e is displays some numbers). You can use this reading to mark your exact position on a paper chart
Essentially a chartplotter is a GPS that can also display charts electronically on its screen, so that it will automatically plot your position onto the chart on the screen.
Having recently installed quite a ncie chartplotter onto my boat, I can say that I am a huge fan. Previously I used a hand held) GPS and paper charts. Having a chartplotter gives me all the information I need immediately. I still carry the hand held GPS and the paper charts as a back-up though.