Wireless Fidelity (Wi-Fi) is a type of wireless network technology used for connecting to the Internet. The frequencies wi-fi works at are 2.4Ghz or 5Ghz, ensure no interference with cellphones, broadcast radio, TV antenna and two-way radios are encountered during transmission.
To simplify, Wi-Fi is basically just radio waves broadcast from a Wi-Fi router, a device detecting and deciphering the waves, and then sending back data to the router. It works very similarly to an AM/ FM radio but it is two-way communication channel. Wi-Fi works over longer distances than Bluetooth or infrared and is also a low power unobtrusive technology, making it suitable for portable devices such as laptops and palmtops. Wi-Fi is governed by the Wi-Fi Alliance, an association of manufacturers and regulators defining standards and certifying products as Wi-Fi compatible.
Source: Wireless Fidelity (Wi-Fi)