Those carbon brushes inside the can connect with the commutator shaft. When voltage is applied through the battery leads to the brushes in contact with the commutator shaft it causes the motor to spin and gives forward and reverse motion to the RC.
Rating a Brushed Motor: Brushed motors are rated by the number of turns of copper wire around the armature within the motor. Stock unmodified brushed motors are usually 20 turn motors but can go all the way down to 7 turn modified brush motors. A higher number of turns provides for more torque but also lower RPMs and less speed. But it also provides longer battery life. The lower the number of turns of the brushed motor, the more voltage can be applied with less torque and higher RPMs -- resulting in more speed.
Pros and Cons of Brushed Motors: They are abundant and relatively inexpensive but the brushes do wear out and the brushed motor will require more maintenance than a brushless motor. Some RC enthusiasts generally recommend a brushed motor for racing (brushless motors are only just now becoming legal in some professional racing circuits) and a brushless for the those who hate maintenance, aren't up for tuning and tweaking, and for backyard bashing -- where the increased dust and dirt can cause havoc for brushed motors.
