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).
Travelers care what wifi is because with wifi, travelers can log on to the internet anywhere and find a hostel, or a map and directions, check email, download music and everything else you do with a computer connected to the internet at home. And iPhones are not the best choice for internet abroad: you'll be happier with a laptop and wifi.
Wifi HotspotsWifi 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.
You can log on to free wifi at hotspots where wifi is intentionally offered to the public without charge; 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.
Searching for unintentionally unprotected wifi which travelers can "borrow" is called by many inventive names, including war driving, which means cruising around in a car with a wifi "sniffer" program, like Mac Stumbler, open on a laptop. Seriously.