Chains transfer 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. Chains are available in various widths 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.