RTR - stands for Ready to Run. Comes with all the electronics such as Remote, Battery and the car. Just load your batteries and go!
KIT - in some case simply "K", example C14K, B24K, it means a KIT that require self assembly, comes with softer tires, 180 size motor, higher torque servo.
* doesn't come with electronics. require own Radio, Receiver, ESC, Battery
KM - KM stands for KIT Metal. Currently the top end of the WPL RC line up. Comes with many essential parts in Metal instead of plastic. Such as Metal Axle Case, Metal Drive Shafts, Metal Steering Knuckle, Dual or Single Speed Gearbox & etc. It's a great out of the box setup that it's cheaper than buying separately and offer much greater performance.
* doesn't come with electronics. require own Radio, Receiver, ESC, Battery
* you can complete the electronics by getting the RES-V3