Not all treadmills are electric. Manual versions of a treadmill work in much the same way as their electric counterparts other than needing to work harder to get the belt to move. This will result in the benefit of additional calories burned.
Treadmills are available in a wide range of designs and prices. Differences may include the strength of the motor, differences in the maximum speed and incline, details on a digital display and the quality of the material that the treadmill is constructed with. It is important to determine the design features that are important for you and test a machine prior to purchasing it. A machine needs to be comfortable for you to use so that you continue your exercise program. When you find a machine that is right for you, it will give you many years of quality exercise.