What is the difference betwen a "messenger" and a "courier"

I've heard the reason why cabbies are such wankers to couriers is that people often used to pay the fare up front and have them deliver 'important' packages and documents. then the couriers took all that work away.

Probably bollocks though. Wigan might know, since he's 72 years old.

Kwik-Fit still do this with tyres.