An extension to 802.11 specification developed by the IEEE for wireless LAN (WLAN) technology. 802.11n builds upon previous 802.11 standards by adding multiple-input multiple-output (MIMO). The additional transmitter and receiver antennas allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity through coding schemes like Alamouti coding. The speed is 100 Mbit/s (even 250 Mbit/s in PHY level), and so up to 4-5 times faster than 802.11g. 802.11n also offers a better operating distance than current networks.
See 802.11 for a complete overview of the 802.11x family of specifications.
Also see the "Wireless LAN Standards chart" in the Quick Reference section of Webopedia.
See also "How Wireless Networks Work" in the Did You Know... section of Webopedia.