The AirPort Express, for its modest size and price, includes several features found in neither a Time Capsule nor an AirPort Extreme, mostly around music. The Express also hides a nifty connection option for extending a network.
AirPlay is a method of streaming media from a computer or iOS device to an external output device, such as an AirPlay-compatible speaker or an Apple TV, or—most interestingly for our purposes—an AirPort Express.
In the case of the AirPort Express, only audio can be streamed. The Express sends the stream through its audio output port (Figure 63) to stereo speakers. You control the settings in AirPort Utility, and then play the audio from an iOS device or from a Mac.