Unlike manual or push lawn mowers, self-propelled mowers have an engine or motor that provide forward and sometimes, reverse motion as well as driving the blades. It is usually the rear wheels that are driven. A self-propelled mower is often denoted by an SP (or PD)in the title. Within the self-propelled category there are a number of different types of geared and gearless drive systems.
Single Speed - A simple type of drive system with one forward speed only, and probably the most common type. The speed is usually set at fairly a comfortable walking pace (3-4 MPH approx'). You can , to a very limited extent, control speed with the throttle but this is not the main function of the single speed drive.
Variable Speed - This is a system that uses a drive pulley system that allows variable speed without gear changing. Basically it works by slipping the drive belt on pulleys. This type of drive is useful and versatile when you are dealing with variable terrain and inclines. This is usually controlled by a switch or lever on the handlebar. Variable drives are sometimes known by other names, often trade names like Shift-On-The-Go.
Hydrostatic - This type of drive converts the power of the engine into hydraulic power using a pump-motor. Again it is a gearless operation, which means you can adjust the speed of the mower infinitely to allow delicate acceleration and enable better coverage and control.