Whenever you communicate information about a control system, you almost always use a block diagram.  Shown below is a prototype block diagram.

In this block diagram, there are a lot of relationships within the system that are shown in a very compressed form.  You need to think in terms of how signals flow through the diagram.

• The input signal is U(s), or just u(t) if you don't want to use Laplace Transform terminology.  The input is the response you want the system to have - often referred to as the desired response.
• The output signal is Y(s), or just y(t).   This is the response of the system.
• The output is measured with a sensor.  In the block diagram that sensor is represented by K_s.  It may be that the sensor just produces a voltage that is proportional to the output.  In that case, K_s is just a constant.  In some other cases there may be dynamics in the sensor (for example, time constant behavior in a thermocouple measuring temperature).  In that situation, K_s may really be some sort of transfer function.  The output of the sensor is often referred to as the measured response.
• The input is compared with the measured response - forming an error signal.  (And the block that forms the error is often called a comparator.)  That error signal is shown in the block diagram as E(s).  Ideally you would like the error to be always zero.  There's no way that is going to happen, but you might be able to reduce it, or you might even be able to make the steady state error become zero (if you use integral control or some variation of integral control).
• The error is acted upon by a controller to produce a control signal (often also called the control effort).  It is this control signal that actually drives the system you are trying to control.
• You could implement the controller with operational amplifiers in an analog control system.
• On the other hand, the controller might be several lines of code in a digital control system.
• The system you are trying to control is represent in the block diagram by a block with a transfer function G(s).  That one block in the block diagram might really be composed of several different parts.
• For example, if you are trying to control the speed of a motor, the G(s) block might contain a power amplifier to drive the motor as well as the motor itself.
• In another example, if you are trying to control the position of an aileron in an airplane, the G(s) contains the aileron, but it also might contain a hydraulic system that actually moves the aileron.
• In these two examples, the power amplifier and the hydraulic system are often called actuators.

• The dynamics of the closed loop system might differ a great deal from the dynamics of the original system you are trying to control.  For example, the system being controlled might be overdamped - showing no oscillations for typical inputs - but the complete (closed loop) system might have severe oscillations.
• While it is possible to get low error, using the control loop above does not guarantee that will happen.