Tethering is a method for connecting a computer to the internet via an internet-capable mobile phone.
"The computer is connected to a mobile phone either with a cable, with a Bluetooth connection or with some other method: the connection to the internet is completed by the phone. Tethering uses mobile applications or software depending on the phone's operating system. Many mobile phones are equipped with software to provide tethered internet access via Bluetooth or a USB cable. Depending on the mobile phone's carrier, tethering may be provided at no extra cost. However, some carriers impose a one time charge to enable tethering, while others do not allow tethering at all or impose additional data charges (examples include the 3 network and T-mobile in the UK) on tethered internet use. Often phone contracts that advertise unlimited or "all you can eat" internet and data usage will bury these hidden charges in their terms and conditions." (http://en.wikipedia.org/wiki/Tethering)