A gear cable is a flexible steel cable which runs from the gear shifter to the derailleur and causes the movement to the gear mechanism to change gears. Because it is such a simple component it is very often overlooked but a badly set up cable can ruin your gear changes and riding enjoyment no matter how much you have spent on shifters and mechs. Just ask anyone who has broken a cable on a ride and they’ll tell you how much of an inconvenience it is.
This simple cable described above is what is known as the inner cable and will need to pass through an outer cable, a plastic coated housing which is reinforced with steel strands, at some point on its journey from shifter to mech. As the shifter pulls the inner cable, the outer cable provides the push against the mech and provides a stiffer channel for the cable to work against which keeps it taught.
The outer cable has ‘cable stops’ made of steel or hard plastic which form a stop against a point on the frame or against a mech. These ‘stops’ stop the outer cable from fraying and also provide a stable base for the outer cable to push against while the inner passes through.