Key kits are sets that contain multiple keys, like feather keys and woodruff keys, in varying different sizes. Keys are machine elements that connect the rotating part of a device to the shaft. They prevent relative rotations between parts and can allow transmission of torque.
In order for keys to function, the part they are connecting to must have a keyway and keyseat, which are a slot and a pocket, respectively. As a whole, the system is called a keyed joint.
Devices that use keyed joints include gears, pulleys, couplings and washers. Essentially, keys are used to facilitate movement between parts.
The two main types of key found in RS key kits are feather keys and woodruff keys.