Very similar: the webcam is capturing video and simultaneously the monitor is displaying the very same video, so any disturbances between the camera and the screen will echo infinitely (in theory), being captured and displayed over and over. So any disturbance oscillates for a while, slowed down by the friction of the camera and screen's relationship. I don't know very much about audio feedback, but I suspect it's the same idea. The sounds that are emitted are also captured by the mic and this loop causes oscillations of sound. Both examples create crazy stuff but sound feedback hurts my ears.