A bike messenger is someone, usually in a large city, who delivers packages, parcels and letters using a bicycle as transport. They do this because bicycles can zip on sidewalks, through traffic and through groups people with a minimum of fuss. As such, a competent 'messer' can deliver a package across a crowded city ad mid day some times as much as 5-time quicker than a delivery company could do with a truck.
It's position I always romanced about, and probably still do if I think hard enough about it. it just seems like the cool life. Whizzing around crowded city streets, zipping though traffic, running over old ladies' feet and bunny-hopping curbs and watching crowds of 7-year olds go "Oooo".
Plus that there is a added benefit that William Gibson wrote a ripping work of fiction featuring bike couriers, Virtual Light. I want a carbon-wrapped paper bike with a beatbox too!