From Team 449 Wiki
A chain is a series of connected links made of either metal or plastic. Since it is generally consistent of several metallic rollers linked together it is used as a connection between a motor and a sprocket so that the motor does not have to be directly attached to the part it is moving.
Since a chain is used to transfer the circular motion of a motor to a sprocket or converting the circular motion to horizontal motion is is generally used to turn wheels, turn turrets, and lift platforms/objects.
This chain is the main type that we use at robotics. This form of chain is special because unlike many conventional ones, these are designed for high wear environments. The design of the roller chain effectively reduces friction, allowing the chain to be more efficient and resulting in less wear to the chain. Furthermore, the roller chain is designed for operation in non-clean environments, and unlike other conventional chains, does not require constant lubrication to keep it working. These chains are specifically designed to fit onto sprockets, so most commonly these chains are used to power wheels (think bicycle).