The AirPort Express is a light piece of hardware that plays your iTunes library via wireless connection.
AirTunes is a complementary technology that will be supported in iTunes 4.6, a free update to Apple's popular music software expected to be released later this week. Connecting AirPort Express into a stereo system using the built-in connector will cause iTunes 4.6 to display a pop-up menu displaying the remote speaker system. Music can then be streamed from iTunes to the stereo.
iTunes Europe speaks for itself.
