Easiest way would be to contrast it with my own vehicle, which is one of the other types. The Silverado only shuts its engine down at intersections. Its electric motor is not large enough to drive the vehicle forward alone. In fact I'm not even sure if its used as an assist. There are really kind of three systems. One just shuts the truck off at intersections but other than that the engine drives the car forward... you don't loose gas idling. It saves you a little bit of gasoline but not a whole lot. Some people balk at using the word hybrid in that system... but too each his own.

The second is a series hybrid. These systems also shut off their engine at intersections. Additionally an electric motor is used to help the engine drive the vehicle forward at low speeds and when extra acceleration is needed. The engine is usually slightly smaller since it doesn't have to provide low end torque. The electric motor is too small to drive the car ahead alone.

The third is a parallel hybrid, which is what my Escape has. In this system a large battery is in the back of the vehicle. In addition to shutting the engine off at idle, the electric motor assists the engine in acceleration AND at low speeds the electric motor is capable of moving the vehicle without starting the engine at all (up to 30-35MPH). The Escape has two electric motors one of which is smaller and is also used as a generator. When stopping the electric motors are used to recapture braking energy and store it in the large battery. The next time the electric motor is used to assist it can use that energy instead of it being wasted as heat on the brakes.

None of the systems require plugging in as the battery is recharged by the engine, and possibly recaptured braking energy.

I think the Silverado is one of the first type of systems (or a combination f the first and second types of systems).... Most of the Hondas are series hybrids and the Escape and the Toyotas are parallel systems.

As you go up the scale you get better and better gas mileage, and less pollution.... with a tradeoff of a more expensive and complex system.

In in the summer in my 4WD Escape I usually average about 30MPG. In the winter that seems to be about 26-27MPG.

Hope that helps.

I'm not sure what the silverado gets someone else here will have a better answer for that.