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).
Recommended Reading: See 802.11 for a complete overview of the 802.11x family of specifications.
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.
Also see Webopedia’s Wireless LAN Standards chart and How Wireless Networks Work.