Cables are basically heavy-duty wires made of many strands of copper wire. An insulating jacket prevents contact with ground or other conductors. Cables have large terminals attached to their ends for connection to the battery, starter, starter relay, starter solenoid or main parts of the electrical system. Cables come in different sizes to match the electrical load of their circuit.
Because of their size, cables are able to handle the large amount of electrical flow (current) needed for the starter and other high-current demands.
Your vehicle's cables should be checked periodically to ensure that they're tight and free from corrosion. Corroded or loose connections can cause a wide range of electrical problems including a no-start, dead battery, and erratic electrical accessory operation. The most important cable connections are the battery terminals, which should be checked at every oil change. The connections should be tight and free from corrosion. The battery case and terminals can be cleaned using a mixture of baking soda and water. As an added measure to fight terminal corrosion, chemically treated felt rings can be placed over the battery posts.
If neglected long enough, cables will need to be replaced. They should always be replaced with a cable of at least the same diameter to minimize the effects of electrical resistance. Changing a battery or other cable may disconnect power to certain memory circuits like radios and other creature comforts such as memory mirrors and seats. If the vehicle's powertrain control module is disconnected, it may need to "re-learn" certain engine operating habits, like idle speed. Check your vehicle's owner's manual for more specifics on what may happen if power is disconnected. Because of the wide variety of technicalities and techniques needed to properly service the electrical system on today's cars, it's best to have your car serviced by a qualified technician.