Road bikes are at home on a smooth tarmac surface. Design features tend to be biased towards speed and efficiency: skinny tyres, 700C wheels, and an aerodynamic forward-leaning position all contribute to going faster.
Pure-bred race bikes (such as the TCR) have drop-handlebars and super - slim tyres with no clearance for mudguards, however there are bikes (such as the Defy and the flat-barred FCR) which make more concessions to commuting with a more upright position, clearances for mudguards and mounts for racks.
Cyclocross bikes (such as the TCX) are designed to go off-road as well as on-road and have clearance for knobbly tyres and the inevitable good old British mud.
Fixed gear bikes (such as the TCR Omnium track) are designed to use a single gear and no freewheel (so you'd better not stop pedalling!!).