Wi-Fi™ is a WLAN (Wireless Local Area Network) technology. It primarily 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 and the Internet.)
There are several versions of 802.11. The original is 802.11b, which provides speeds up to 11 Mbps. 802.11g and 802.a are faster versions. Most 802.11g and 802.11a products are backward-compatible with the original 802.11b.
Even faster is the newer 802.11n version. Faster still is the even newer 802.11ac standard. Like 802.11g, these are based on the original 802.11b and backward-compatible.
Wi-Fi is generally much faster than data technologies operating over the cellular network like GPRS, EDGE, 1xRTT, HSDPA, and EV-DO.
It is much shorter-range, however. Wi-Fi coverage is only provided in small, specific areas called "hot spots". Other than some corporate or educational campuses, Wi-Fi coverage is not widespread. Range for a typical Wi-Fi base station (access point) is typically around 100 to 300 feet indoors and up to 2000 feet outdoors.
Most Wi-Fi operates in the 2.4 GHz unlicensed frequency band. This is the same band as Bluetooth and some cordless phones, although the technologies are designed to co-exist and not interfere.
802.11a and 802.11ac operate in the 5 GHz unlicensed frequency band. Since the 5 GHz band is often not as "crowded" as the 2.4 GHz band, it can be faster in many cases.
Wi-Fi networks can be set up and operated by anyone, with different networks allowing different kinds of access. A public "hot spot" at an airport or coffee shop might charge an hourly rate for access. A hotel might offer free wi-fi to guests. A company or university might offer on-premises free access for verified employees/students. Or a home user could set up their own network to which only they had access.
While most Wi-Fi connections are between a mobile device and an access point, it is also possible to create an "ad-hoc" network directly among two or more devices, without an access point.
Wi-Fi is a trademark of the Wi-Fi Alliance, a nonprofit industry association. The IEEE technical specification for Wi-Fi is 802.11. "Wi-Fi" isn't short for anything. The "Wi" part is suggestive of "wireless" and the whole term is suggestive of "hi-fi", but "Wi-Fi" is technically its own trademark and not an acronym, nor an abbreviation.
On this site, phones with UMA may not always have Wi-Fi also listed as a feature, even though UMA may technically use Wi-Fi, since UMA is a very specific and limited implementation of Wi-Fi technology.