"Wifi" means "wireless fidelity". The term "wifi" refers to certain kinds of wireless local area networks, or WLAN (as opposed to LAN, or computers that are networked together with wires).
Travelers with PDA's (like Blackberries) and other handheld devices or laptops with wireless cards can connect to the internet via wifi. A wireless card is like a modem without a phone line (like an airport card in a Mac).
Wi-Fi is short for wireless fidelity. It refers to a local area network which uses high frequency radio signals to send and receive data over short distances (approx. 200 feet).
Wi-fi is also written as wifi, WiFI or even WIFI.
Wi-Fi is also the trademark of the Wi-Fi Alliance, the trade organization that defines the Wi-Fi standards.
Wi-Fi or wireless is used to refer to communicating without cords or cables. It mainly refers to using radio frequencies and/or infrared waves.