Servo systems consist of four main components — motor, drive, controller, and feedback. In some cases, a standalone controller determines required motor moves and prompts the drive to supply the necessary electrical energy to make those moves happen. In other cases, drive and controller are integrated into one component. Either way, the drive core controls torque or velocity or position ... although in servo systems, the most common command parameter is torque. That control is via a torque-mode amplifier or linear drive function.
Note that servo drives are also sometimes called amplifiers because they take control signals from the controller and amplify them to deliver some voltage and current to the motor. These are not to be confused with integrated designs (including motor, feedback, controller, and drive) also sometimes called drives for their axis-driving function.