weedpharma:
The physical key is usually in the fob and released by a button. It only lets you in the door. You still need the fob to be able to start the car due to the ID chip in it.Weedpharma
There are variants of how they work.
But loose or get something wrong and the remedy can be very very expensive.
You may have a spare , but if its at home and you are travelling abroad make sure you have tow home insurance.
It can be cheaper than a new EMU. 
Not me but happened to a mate.
westfw:
Great for women - throw the fob in your purse and forget about it.
So that's where my cc went.