I think he's referring to a
Stepper Motor.
MicroMo is one company that sells them along with stepper motor controllers. It's basically a motor that has 2 (+) inputs and 1 (-). As you alternate between the two (+) inputs the motor completes a fraction of a revolution. You use it like a regular motor, but the speed is determined by an external controller and how fast it switches between the inputs.
For example, if you get a stepper motor with a 30 degree resolution, every time you alternate to the other input, the output armature moves 30 degrees. Coupled with a gear reduction, these motors can be made to be very accurate. However, with these motors you need a potentiometer for positioning to determine electronically to when the motor has reached its next step. If you switch early it will go back to the previous step.
If you haven't purchased it yet, then I think it might be easier to use a
servo motor for arm positioning. Although they cost a lot more, it's a lot easier to use. The position is determined by how much power you send it.
Basically:
If cost isn't a factor and time is, go with a servo motor.
If cost is a factor and time isn't, go with the stepper motor.
Whatever you get, you'll need a motor that can hold ~112N if your mass is 25lb.