Headphones. Actually they are properly called In-Ear-Monitors. Everyone in the band wears them for our performances and most of us use them to listen to music on our iPods or computers as well. Good isolation is the key. Isolation is very important on stage and very useful on the bus or in an airport or airplane – whenever your surroundings are noisy. The better the isolation, the less you have to turn up the volume to hear the music.
Link to the page for SCL5s on the Shure website.
And apropos ears, check out this item from the BBC:
Vincent van Gogh did not cut off his own ear but lost it in a fight with fellow artist Paul Gauguin in a row outside a brothel, it has been claimed.