Im not sure what you are asking for. Can you specify a little bit more? Op amps, transistors, what are you interested in knowing?
Feedback, especially negative feedback is very common in electronics. Think Op-Amp and other differensial amplifiers. In a circuit with lots of gain and good bandwidth.. Then negative feedback is used to scale down the gain. It increase the stability and makes it possible to get high bandwidth and less noise - linearity. It's also easier to compensate an input and output of an amp with negative feedback (depends if you want current or voltage in/out)
Positive feedback is being used for different reasons. Mainly in oscillators and comparators/schmitt triggers.
Transistors can be feedback biased too. Read more about them here:
http://www.allaboutcircuits.com/vol_3/chpt_4/10.html