click on the title of this post, and you'll get directed to part of the excellent Fun With Transistors Page on amplifier feedback. feedback can be used to increase linearity of the output, curing oscillation, reducing gain, and tonal shaping among other things when used in amplifier stages. usually when we speak about feedback, we are talking about negative feedback in amplifiers. this is accomplished by adding an out of phase signal to the input of a gain stage, often by taking the output of an inverting amplifier. the webpage takes an in depth look at different types of feedback complete with diagrams and equations for those that are so inclined. check out the rest of the site as well as the Fun With Tubes page which has some great info, too.