A beautiful and awsome addition to cars , smart keys ...
I need to set a clear definition for the following terms 
keyless go - key less entry - proximity - smart key - IR keys - push to start 
 
I - Keyless go 
A system that enables the driver to lock and unlock the car without using keys ..or pushing remote buttons! 
so, as long as you carry the {smart key} when you pull the car door handle the car automatically unlocks  and open the door . door handles got sensors , and some has buttons, you push the button and the door unlocks if you carry the smart key

II - key-less entry 
Unlocking the car without a key, using a remote, pressing the remote's button  to lock / unlock the car. 

III - Proximity 
Proximity sensors are 
 Proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.  Wikipedia
So I guess it's a term that refers to keyless Go systems 

IV - Smart Keys 
smart keys are sweet .. I love 'em , it's just opening the car and turning the ignition on without using a key, the metal key, the blade itself .
in some cars u'll find a button instead, and the "smart key" is placed in a dock, where the car can read it's code and if it the correct smart-key, it'll start . 
smart keys has different shapes, there are card like keys, as in renault and mazda , there are some nicely shaped fobs as in the merceds or BMW smart keys. let's quote something from Wikipedia : 
 
Smart key is an electronic access and authorization system which is available as an option or standard in several cars. It was first used by Mercedes-Benz in 1998.  Wikipedia
For the record, mercedes benz always do things first :) . 

 
V- IR Keys
I don't know much cars that used IR keys in thier systems, just Mercedes benz,  these are Keys that uses Infra Red to transmit signals between the car and the key. 
 
VI- Push To start : 
look at the picture : 

 
this video should be convenient :