The metal interwoven flex that operates mechanical brakes such as disk brakes and V brakes .

Positive

• Flexable (easy to bend to run smoothly with the frame)
• Cheap (every bike shop sells cables)
• Low maintanence, unlike hydraulic systems, you obviously never need bleed the system.
• Ease of adjustability, you can adjust the brakes by simply tightening the cable at the lever.
• No risk of contaminating the brake surfaces with brake oil.

Negative

Ends fray after time. This can be prevented in the first place by clamping the nipples provided onto the end of the cable. If you don't have nipples then you can use super glue as a temporary fix.