A term used in automotive circles to describe the components comprising the rear axle of an automobile, especially the differential in RWD cars. In FWD cars, one would probably refer to the said components as the "front end".
A rear end ratio is the effective gear ratio of the differential. A higher rear end ratio provides better acceleration at the cost of top speed and fuel economy. Typical rear end ratios include 2.53:1, 3.07:1, 3.53:1, 4.11:1, and 5.06:1