Bluetooth headset beginner's question?

I see those things all the time,but what are they? Is it a phone that you wear on your ear? I tried searching Wikipedia,and googeld ....but nothing just explains what they are and just gives a basic explanation.

You talk through them. Your turn your bluetooth on in your phone and put the headset on your ear.. Once you get a call you push the button on the headset and you talk through it.

So now you can use two hands while driving.