There are several different types of motors. Each motor type has several advantages as well as disadvantages depending on a particular robots design. See this Wikipedia article for basic information about the different electro motors.
There are several different types of AC-motors, but their use is limited to high power stationary industrial robots. They are harder to use than DC-motors.
DC-motors are very easy to use, but like most other motors their usefulness for robotics is very dependent on the gearing available. DC-motors are made much more effective if they have an efficient gear ratio for a particular task. If your priority is to have a fast spinning motor and torque is of little concern a low gearing or even no gearing may be what you need; however, most motors used in robots need torque over top speed so a motor with a high gear ratio could be more useful.
The control of a DC motor can be split into two parts: speed and direction.
Changing which direction a DC-motor turns is very simple: simply reverse the polarity.
Both pairs of switches ( (S1A,S1B) and (S2A, S2B) )-see the picture on the right- will always switch together. This circuit is called an H-bridge. In a real design the switches can be several different components (Relays, transistors, FETs) or the whole circuit (without the motor) could be an IC (integrated circuit. use sugarcan relays
Speed is a little bit more complicated. Many beginners would try to slow down a motor by reducing its voltage with a variable resistor or other ways. This does not work well, because it will not only reduce the motor's speed, it will also reduce a motor's strength, while also consuming a lot of electricity as large amounts of heat are generated by the resistor.
A far better way is to use a PWM (Pulse-width modulation) device.
Servos are used in robotics for different uses: e.g. to move a sensor around, or to move the legs of a robot. Some users modify the servo so they can use it as a DC-motor with a gearbox.
Controlling a servo is done with Pulse-width modulation. The length of the pulse is relative to the position the servo will turn to. The length of this pulses is usually located between 1ms and 2ms, if so 1.5ms would be the center position. This pulse needs to be repeated with small intervals (otherwise the servo might turn to a "save" position or it might simply stay at its current position. This depends on the type of servo used).
Stepper motor 
We cover stepper motors in more detail in the next chapter, Robotics/Stepper Motors.
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