http://www.visionengineer.com/aero/nozzle.shtml
Nice simple explanation Keith, See above link for a diagram.
(The only extra clarification I can add, is that in the Con-Di nozzle the exhaust flow is always accelerating.
The acceleration in the convergent part is when the exhaust flow is less than mach 1.
At the narrowest part of the nozzle, the flow is at Mach 1
And at the Divergent part the flow is accelerating beyond Mach 1
Hope all this helps.