Bluetooth is just a wireless connection protocol made to be plug and play for low and med-bandwidth devices. Sort of competes with wireless USB (yet to come out). Lots of phones (like the RAZR I have) have built-in bluetooth so you can connect to a wireless handsfree set. When a car has Bluetooth, it usually means that the car's stereo can act as a bluetooth headset for your phone, so you don't have to hold it while you're driving. On computers, it can be used for lots of things, similar to plugging devices into your USB ports (of course, the device has to support Bluetooth to do it).
Eventually USB plugs will be completely replaced by Bluetooth and/or wireless USB devices. Just walk up to your computer and your Wireless USB disk will connect, etc, etc.
http://en.wikipedia.org/wiki/Bluetooth
http://en.wikipedia.org/wiki/Wireless_USB