A bicycle chain is a 'roller chain' that has the task of transferring power from the cranks to the rear wheel of the bike thus propelling it. Most bicycle chains are made from alloy steel, but some are chrome plated or stainless steel to prevent rust, or simply for good looks. Chains will vary in width depending on the amount of gears you have, most are 3/32 inch for use with geared systems and then come in 'speeds' which needs to matched to the cassette, ie a 10 speed cassette will need a 10 speed chain and so on. Track bikes or single speeds often run 1/8 inch rings so will need a 1/8 inch chain as well, these will have larger plates to aid stopping on a fixed wheel, which can add extra stress to a chain.