AirPort Express is a networking device to wirelessly deliver music from a Mac or Windows machine to a connected sound system using AirTunes technology. It also serves as a WLAN access point, can be used as a (USB) print server and can be connected directly to a DSL or Cable modem or an existing LAN.
See http://www.apple.com/airportexpress/
Seattle Wireless has some info on the underlying hardware here: http://www.seattlewireless.net/index.cgi/AppleAirportExpress FCC’s photos of the motherboard (PDF format): https://gullfoss2.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachment_id=428763&native_or_pdf=pdf
The main chip on the motherboard, from Broadcom, contains a MIPS CPU, wireless and ethernet controllers, and more.
Photos of the motherboard show, near the center of the board, three bare contacts which might or might not implement a serial console device for the underlying operating system, rumored to be Wind River’s VxWorks (a tiny and somewhat unixy embedded OS).