Wi-Fi is a WLAN (Wireless Local Area Network) technology. It provides short-range wireless high-speed data connections between mobile data devices (such as laptops, PDAs or phones) and nearby Wi-Fi access points (special hardware connected to a wired network).
The most common variant of Wi-Fi is 802.11g, which is capable of providing speeds of up to 54Mbps and is backwards compatible with 802.11b (providing up to 11Mbps).
There is currently a new standard in the works called 802.11n (offering twice the speeds of 802.11b) and there are already retail networking devices that support its draft specifications. Currently, there are no mobile phones that support the draft 802.11n specifications.
Wi-Fi is much faster than any data technologies operating through the cellular network like GPRS, EDGE and even UMTS and HSDPA.
The range covered by a Wi-Fi access point is from 30 to 100 meters indoors while outdoors a single access point can cover about 650 meters.