Feedback is a process whereby information from an output of a signal is used (intentionally or unintentionally) to alter the input of the same signal. Commonly used in operational amplifiers. An everyday example is the high-pitched audio frequency heard when a microphone picks up audio from its own amplified output.
There are two basic types of feedback: Negative and positive.
Negative Feedback: When the output is used to attenuate or diminish the input signal. Commonly used to correct errors or limit levels to an acceptable value. Negative feedback occurs when the output is between 90 and 270 degrees out of phase with respect to the input signal.
Positive Feedback: When the output is used to amplify or enhance the input signal. Can be used to decrease sensitivity to small signals or noise. Positive feedback occurs when the signal is in phase with the input signal.