I had a feeling that would be your response James, so I refrained from making pretty much the same suggestion
In it's simplest form, a sound font can be regarded like a collection of pre-built 'voices'.
They are complex sample banks that the soundfont player scans when it gets the appropriate MIDI trigger. Want a different 'instrument' - switch soundfont. Good soundfonts with a good player will give different responses to different pitches and key velocities, just like a real instrument would.
I'm sure someone else could explain this better!