What is the difference between a single shaft and dual shaft motor?

Quite simply a single shaft motor has one shaft which comes out of the face of the motor and a dual shaft motor has the shaft coming out of the face and rear of the motor. The single shaft is a straight forward concept which can be easily understood. The dual shaft can be used in a number of ways. One example is to drive 2 applications on either side of the motor. A second is to use the rear shaft to mount an encoder to actively monitor the actual speed of the motor and then make adjustments as required.