DUN is a Bluetooth profile (mode) that is designed to link an Internet-enabled device such as mobile phone to a device such as a laptop or PDA so it can access the Internet.
DUN will therefore allow you to connect you Bluetooth-enabled laptop to the Internet via your Bluetooth-enabled phone and wireless service. Because it works over Bluetooth, the link is wireless, so there is no cable; you can leave your phone in your pocket or purse.