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 older variant of Wi-Fi, 802.11g, is capable of providing speeds of up to 54Mbps and is backwards compatible with 802.11b (providing up to 11Mbps).
The more recent standard is called 802.11n (offering speeds of up to 150Mbps per channel or up to 600Mbps in total). It can be used in the 2.4 GHz or 5 GHz frequency bands, though a receiver needs to have dual-band antenna to operate on both.
2013 saw the birth of the latest (so far) Wi-Fi ac standard. It allows for speeds of up to 500 Mbps per channel and over 1Gbps in total. Wi-Fi 802.11ac operates only on the 5GHz band.
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.