what tire is it? rubber?
what is it on? concret?
My background is in bike tires.
Fundamentally, it is the amount of energy required to deform your vehicle's tire as it spins on a surface. If you spin a wheel in the air, there is no deformation, thus no rolling resistance.
Some of this energy is released as sound. Car tires are often louder than the engine. Much of the energy is turned into heat. Motorcycles preheat their tires before races to achieve consistent behavior throughout the start transient of the race.
The flexibility of the sidewall of a tire also plays a big part. The softer and more flexible the sidewall of a tire is, the less rolling resistance it will have. The stiffer and more rigid the sidewalls, the more resistance to deformation.
In both cases, the tires still deform the same amount. On my bicycle, let's say I have the tires pumped up to 100psi and the total weight of myself and the bike is 200 pounds. This means that the tires will always deform such that there are 2 square inches of rubber contacting the ground -- regardless of the stiffness of the sidewall. That means that our deformation is constant, and the only thing that changes is how much energy is required to get the tire into the deformed shape.
The tread will also come into play. The more knobs you have, the more they have to bent around as they come in and out of contact with the ground.
Hope that helps :D