The small DC motor, is likely to use more power than an Arduino digital output can handle directly. If we tried to connect the motor straight to an Arduino pin, there is a good chance that it could damage the Arduino.
A small transistor like the PN2222 can be used as a switch that uses just a little current from the Arduino digital output to control the much bigger current of the motor.
The transistor has three leads. Most of the electricity flows from the Collector to the Emitter, but this will only happen if a small amount is flowing into the Base connection. This small current is supplied by the Arduino digital output.
The diagram below is called a schematic diagram. Like a breadboard layout, it is a way of showing how the parts of an electronic project are connected together.
For more detail: Arduino Lesson 13. DC Motors
French 
English