Bluetooth is a low-power, low-bandwidth wireless connectivity specification.
Common uses include:
Bluetooth cellphone automatically syncing to your PC (when equipped with a Bluetooth receiver) when you walk into the room.
Bluetooth wireless keyboard/mouse (Microsoft sells a set).
Bluetooth cellphone with a wireless Bluetooth headset (Jabra makes a couple, I'm not sure who else).
These are what I can think of off the top of my head, though I'm sure someone else can chime in.
Is there something in particular you were thinking you might want to use it for, or were you just curious about the tech in general?