RF Proximity Continuous Transmitter and Receiver

Hello everyone, I am an absolute newbie to component level engineering, but I do have experience in telecommunications and some programming languages...

What I am trying to do is, sort of like when you have a key fob in your pocket and you go to your car and it unlocks automatically and even lets you start the car by push button... You don't actually need to push any buttons or use any keys, because you have that coded sensor, key fob in your pocket (or purse).

I assume that this is done via RF communication and proximity (calculated by db levels), but I don't know where to start to find components like these for my Arduino.

Anybody have any ideas on what kind of sensors I am looking for, and where to find them? I guess it would also use the same concept as a Dog's Invisible Fence.

Thank you in advance for any help.

Are you talking about RFID?