What's feedback?
How do I control it?
I know nothing about it except the name, sorry.

I was listening to Rage Against the Machine the other day and I noticed Voice of the Voiceless intro sounds... weird.
How does Tom do that?
He said something about feedback in an interview...
I've never listened to the song but feedback is the sound you here when you put a mic too close to an amp or speaker when the mic is turned up to high

its that really loud high pitched sound thats used in movies for some reason every time someone steps up to a mic it seems
Its high pitched squeals. It happens when a mic picks up sound from an output and sends it back through the output and thus picks it up again and sends it through again infinitely.

You can do it with an electric guitar by holding it up to an amps speaker or by having it up real loud and simply turn towards the amp. Im not sure, but I think with guitar it happens when the strings are vibrated by the amps sound.
"Good and evil lay side by side as electric love penetrates the sky"

Basically in the context of an instrument it's the sound that is picked up by you're pickups being sent to your amp, being amplified by your amp and then being picked up again by your pickups thus creating a loop and "feeding back". Turn your amp up as loud as it goes, pluck a note or a chord or hit your guitar with a hammer whatever you want just make some noise, now stick your guitar right up to the speaker cab. Unless your just using a practice amp or your amp is seriously clean you will get feedback. Now controlling it is another matter. I don't think you really can control it. You can manipulate it, and Tom Morello does all the time as well as Hendrix did and some music is built off of it completely, some BORIS springs to mind, but completely controlling it is futile. Morello uses his killswitch and whammy bar to manipulate it mostly and using effects will give it different tonalities. Voices of the Voiceless intro just sounds like flanger, Digitech Whammy pedal and the guitar slightly feedbacking to me. I'm not Tom though.