A mountain bike chain is one of the most important components of a bicycle. It is the main piece that transfers power from the rider to the wheel. On a mountain bike the chain is tensioned by the rear derailleur which allows the chain to shift from one cog to another changing the gear ratio and thus making the pedals spin more or less compared with the wheel.
A mountain bike chain is comprised of small figure eight shaped pieces with cylinders running through the center of the holes creating an adequate space for the gears and chain rings to grip. Each pieces can rotate a certain amount and when combined form a complete loop.
The construction of a mountain bike chain allows the rider to fix the chain by taking out the broken pieces and shortening the length of the chain. This is not possible on single speed bikes because there is no derailleur tensioning the chain.