Keyboard or Synthesiser

by Nis
(UK)

Hi

I'm a thinking of learning to play the keyboard and would like to know what the difference is between a keyboard and a synthesiser. For example, i have seen the demos of the yamaha psr500 keyboard and the yamaha mm6 synthesiser and the latter seemed more 'advanced'. Could somebody help me out with what a keyboard does which a synthesisr can't and vice versa. Put differently, can you learn to play the keyboard on a synthesiser?