Adding a motor to a circuit is, from a design perspective, similar adding an inductor. This is becasue a typical DC motor consists of an electromagnetic coil surrounded by permanent magnets. When current flows through the coil a magnetic force is induced around it an it pushes back against the magnetic field of the surrounding magnets.
The simplest type of motor you're likely to come across is a DC Motor
There are quite a few different parts in even the cheapest motor, but for the sake of simplicity we can consider just the most important parts. A DC motor consists of a permanent magnet, an electromagnet and a housing to constraion them both.
###Rotor
The "inside bit" that rotates is called the rotor. This tpyically consists of a copper coil that functions as an electo-magnet. DC motors where the outer casing rotates are known as out runner motors and are typically found in very high speed devices such as quadcopters.
###Stator
Surrounding the rotor is the "outside bit", or that stator, which is a stationary component. This consists of the case of the motor and more magnets, which can be either permanent or electro-magnets.
###H-Bridge
Even the smallest DC motors are capable of consuming quite a bit of current. Since the Arduino is only capable of supplying as mere 20mA of current we are going to need some external circuitry, such as a h-bridge.
<iframe frameborder='0' height='448' marginheight='0' marginwidth='0' scrolling='no' src='https://circuits.io/circuits/2467321-lab-3-motor-control-with-h-bridge/embed#breadboard' width='650'></iframe>At the other end of the motor spectrum are stepper motors. These are designed for applications that require higher levels of precision. As a rule of thumb, more expensive steppers will be capable of higher and higher levels of precision and higher loads.
A servo motor adds a small gearbox and a potentiometer to the motor which allows for finer control over its rotational position. In many servos, especially those aimed at hobbyists, the servo is limited to less then 360 degrees of rotation. It is possible, however, to purchase continuous rotation servos. Like a basic DC motor as servo has inputs, generally red and black, for Voltage and Ground but it also has a data input, typically yellow or orange, which allows the user to control its angular position.
Arduino comes with libraries for extending its functionality, and making it easier to connect to a variety of standard components.
The code above calls for the servo to be attached to pin 9. A schematic of the circuit is included below [click to view larger image]
