A vibrating motor is essentially a motor that is improperly balanced. In other words, there is an off-centred weight attached to the motor's rotational shaft that causes the motor to wobble. 
The amount of wobble can be changed by the amount of weight that you attach, the weight's distance from the shaft, and the speed at which the motor spins.

This type of motor can be affixed to all kinds of objects, which will cause them to vibrate and move freely about. This is a quick and dirty way to get a Simple Bot to move about, but not exactly the most elegant. Vibrating motors can be found inside cell phones, pagers, gaming controllers, and personal massagers.
In absence of those, you can easily build your own vibrating motor by attaching any off centred weight to any motor shaft. They can also be created by breaking in half balanced components already attached to motor shafts.