Road brakes are also known as caliper brakes. They come fitted as standard to all road bikes and some commuting bikes. They can be indentified by there simple one bolt attatchment to the bike with the cable pulling from just one side. Popular manufacturers include Shimano, Sram and Campagnolo.
V-brakes come fitted on mountain bikes and commuting bikes. They are easily identifiable by the n shape they make. The arms move parrell to each other, either side of the wheel. The length of the arm can vary and there are mini and standard verisons available. These pads will fit both.
Cantilever brakes come fitted as standard on most cyclo-cross bikes and older mountian and commuting bikes. They pull up using a straddle wire and the wire and brakes form a diamond shape when looked at head on.