well, inasmuch as not all couriers ride the same bikes...
yes, with a massive degree of generalisation, they are track bikes, often modified for the road by addition of brakes/stronger wheels/different handlebars.
sometimes they are purpose-built street bikes with more relaxed road-style frame geometry, but with track dropouts to cope with a fixed wheel.
well, inasmuch as not all couriers ride the same bikes...
yes, with a massive degree of generalisation, they are track bikes, often modified for the road by addition of brakes/stronger wheels/different handlebars.
sometimes they are purpose-built street bikes with more relaxed road-style frame geometry, but with track dropouts to cope with a fixed wheel.