I'll just throw in some extra examples to compare, I think Sonia's answer is pretty clear. :)
I have a car.
- Ok, we understand that.
I had a car.
- Once, in the past. I don't have it anymore. Simple Past.
I have had this car for ten years.
- ...and I still have it now. Present Perfect.
My car broke down. I had had it for ten years.
- Two events: owning the car and the car's breakdown. Using Past Perfect "I had had..." means owning the car happened before the Simple Past sentence about the breakdown. Even though the Past Perfect sentence comes second, it is clearly not the most recent event. How cool is that grammar!