It's a somewhat gimmicky but potentially amazing approach to musical performance: you make a noise into a magic box which records the noise and plays it back repeatedly, then you make another noise and another, and pretty soon you've piled up several noises and they are playing at the same time and you didn't have to grow extra arms or clone yourself.
Here are a couple of folks who have really made some cool noises using this technique:
Casey Driessen: http://www.youtube.com/watch?v=v9GyoUmhKU0
Walk Off The Earth: http://www.youtube.com/watch?v=tJ8nkJ1_Ee0
These folks use big piles of nifty gear, but I didn't want to invest in a piece of equipment until I had the chance to experiment with loopiness. Enter Mobius: http://www.circularlabs.com. Free (donations accepted), robust looping software. I got it running on my trusty little workhorse of a netbook (along with my USB audio interface), and quickly noted 2 things: 1. Mobius is awesome, and 2. there is no practical way to use it for live looping without foot pedals. So I decided to make them.