"WiFi" means "wireless fidelity". The term "wifi" refers to certain kinds of wireless local area networks.
WiFi is becoming increasingly popular amongst travelers as an instant way to get online via WiFi hotspots.
Wifi hotspots are places where you can find WiFi, free or paid. Internet cafes are likely WiFi hotspots, and many airports, hotels and bars have WiFi hotspots.
Some WiFi networks are protected with passwords and you must pay or otherwise be given access to log on. Generally, you can log on to paid WiFi with a credit card online; your screen may open with a splash page for the WiFi provider, offering you payment choices, if you are trying to log on to the internet in a paid WiFi hotspot.