There are three main type of blowers: Multi-Speed, High-Efficiency Multi-Speed, and Variable Speed.
Standard Multi-Speed blowers have different speed options and you select the speed that the blower will operate at depending on the function it is performing (ex. different speeds for fan, AC, and heating functions).
High-Efficiency Multi-Speed blowers are designed to maintain a constant torque and in doing so will maintain air-flow and use less electricity.
Variable Speed blowers will adjust their speed during operation. The result is a blower that will run at lower speeds for longer periods of time (this improves the air flow and increases electrical efficiency).