Concept of Feedback:
It is used most commonly in an amplifier. Part of the output from the amplifier is fed back to the input and this concept is called feedback.
When the input signal of amplifier and part of the output are In-phase with each other, it is called as Positive feedback.
When the input signal of amplifier and part of the output are Out-phase with each other, it is called as Negative feedback.
The concept of positive feedback results in oscillations, so they are used to generate oscillations of particular frequency as required. Such circuits are called as Oscillators.
Good information about feedback.