Linear actuators move objects in a straight line, usually in a reciprocating motion. Rotating the actuator is to let the object rotate at an angle, which can be any value. Therefore, the linear actuator performs a reciprocating motion, and the rotary actuator performs a rotational motion.
Simply put, a single motor is a rotary actuator. When a current flows through a conventional servo motor, the motor rotates. Directly connecting the motor to the load constitutes a direct drive rotary actuator. Many rotary actuators are combined with mechanical devices to form a mechanical lever structure that helps to reduce rotation and increase torque. If the component ultimately performs a rotational motion, it also belongs to a rotary actuator.
On the other hand, it is also possible to convert the rotary motion into a reciprocating motion by connecting the rotary actuator to the mechanical device, and then it becomes a linear actuator.