As far as control system is concerned, feedback is defined as a control loop which can either enhance or attenuate the output signal. Enhancement is done with the help of positive feedback, while reduction in error is done with the help of negative feedback.
Positive feedback can be defined as that type of feedback which amplifies the signal. Under certain gain conditions, positive feedback reinforces the input signal to the point where the output of the device oscillates between its maximum and minimum possible states. They occur when the fed-back signal is in phase with the input signal. Positive feedback may also introduce hysteresis into a circuit. This can cause the circuit to ignore small signals and respond only to large ones. It is sometimes used to eliminate noise from a digital signal. It is also known as a self-reinforcing loop. Any process can be influenced by positive feedback can increase or decrease its output until it attains the prescribed level.
Negative feedback exists when the fed-back output signal is out of phase with the input signal. This occurs when the fed-back signal is anywhere from 90° to 270° with respect to the input signal. Negative feedback is generally used to correct output errors or to lower device output gain to a pre-determined level. It also tends to reduce the input signal that caused it, is also known as a self-correcting or balancing loop.