There's the radiator, and there's also the coolant reservoir.
engine coolant temp sensor.
I believe that you fill it with coolant, as the tank's name suggests.
This is used to keep the motor from overheating and from freezing. Thus the name "antifreeze" and "coolant".
madarchod
It is a coolant temperature sensor.
hydrometer
Toyota Long Life coolant. Pricey. Any brand name ethylene-glycol based coolant, mixed 50/50 would be OK.
It could be the coolant overflow tank that is leaking.
Yes, If to much leaks out this will cause the engine to over heat, smoke and or stall hence the name engine COOLant :)
Ethylene glycol. Pretty much any coolant that doesn't have a different name plastered on the container (like Dex-Cool).
Ordinary light water