Wi-Fi® is a WLAN (Wireless Local Area Network) technology. It primarily provides short-range, high-speed data connections between mobile data devices (such as laptops) and nearby Wi-Fi access points (special hardware connected to a wired network).
Wi-Fi is a trademark of the Wi-Fi Alliance, a nonprofit industry association. The IEEE technical specification for Wi-Fi is 802.11.
There are several variants of 802.11. The most common is 802.11b, which provides speeds up to 11 mbs. 802.11g and 802.a are faster versions. Many 802.11g and 802.11a products are backward-compatible with the original 802.11b.
Range for Wi-Fi is typically around 100 to 300 feet indoors and 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 operates in the 5 GHz unlicensed frequency band.
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.