Just a quick example for you:
I have a Bluetooth (BT) cell phone and a BT USB adapter for my desktop( much like the 802.11 adapter). What this combination lets me do is syncronize my Outlook contacts with my cell phone wirelessly. The phone doesn't contain all the information on a contact, but the important ones (Name, home, work, cell and e-mail) get transferred.
(With the right software, you can use your computer keyboard to type SMS messages on your phone. You can also get your cell phone to control your computer. Winamp is apparently a favorite)
When I get into the car, the phone automatically syncs up with the car. When I get a phone call, the radio automatically mutes and the call goes through the stereo system. The microphone in the car transmits my voice.
All of this without plugging the cell phone into anything. It stays in my pocket.