What makes a cellphone vibrate?

What makes a cellphone vibrate? Is it a central mechanism, or are there parts located throughout the phone?

Among the many components inside the phone is a small motor. 

The motor is built in such a way that it is partially off-balanced.

In other words, a mass of improper weight distribution is attached to the motor's shaft/axis. So when the motor rotates, the irregular weight causes the phone to vibrate.

Below is the image of one type of vibrating motor; there are many types.

The size is small... smaller than many currency coins.

The mass at the left end in the picture below, almost resembling a half-moon is the off-balanced weight, the cylindrical part contains the motor.

More phone motor detail,you can browse: www.vibrationmotor.net for reference.

With love