http://www.bluetooth.com/bluetooth/
BlueTooth is kind of like USB. In the sense that there are BlueTooth mic, keyboards, headsets, head phones, printers, cameras, remotes, and more. Your iphone can be paired to your computer to be used as a fax modem, make phone calls, and in some countries share internet. If you pair a razor to your mac, you can send file to and from the phone.
Bluetooth is harder to develop for then USB. Blue tooth is rather slow compared to say wifi. blue tooth is designed to be one to one. a usb device can only be paired to one computer at a time.
Apple developer website can tell you about developing for blue tooth devices for iphone or mac os x.
