You can combine the two power sources found in a hybrid car in different ways. One way, known as a parallel hybrid, has a fuel tank that supplies gasoline to the engine and a set of batteries that supplies power to the electric motor. Both the engine and the electric motor can turn the transmission at the same time, and the transmission then turns the wheels.
The animation below shows a typical parallel hybrid. You'll notice that the fuel tank and gas engine connect to the transmission. The batteries and electric motor also connect to the transmission independently. As a result, in a parallel hybrid, both the electric motor and the gas engine can provide propulsion power.

By contrast, in a series hybrid (below), the gasoline engine turns a generator, and the generator can either charge the batteries or power an electric motor that drives the transmission. Thus, the gasoline engine never directly powers the vehicle.

Take a look at the diagram of the series hybrid, starting with the fuel tank, and you'll see that all of the components form a line that eventually connects with the transmission.

3) Power Assist and Engine Downsizing
The most basic definition of a hybrid vehicle is one that uses two methods of providing power to the wheels. As a result, the ability of an electric motor to help share the load with a gasoline engine is the technology step that, on top of the first two, truly qualifies a vehicle as a hybrid. A vehicle meets this classification only if it has a large enough motor and battery pack such that the motor can actually supplement the engine to help accelerate the vehicle while driving. This power assist ability, combined with downsizing the engine, allows the vehicle to achieve the same performance as a vehicle with a larger engine while achieving superior fuel economy. Typically vehicles containing these first three features are categorized as a "mild" hybrid like the Insight, Civic, and Accord hybrids from Honda.

4) Electric-only-drive
This technology step allows the vehicle to drive using only the electric motor and battery pack, thus taking full advantage of electric side of the dual system. With this step, we separate out "full" hybrids such as the Toyota Prius and Ford Escape Hybrid. This is the reason why Prius owners are sometimes shocked when they start their car and don't even realize it's on—only the quiet battery system is operating the car rather than the traditional rumble of the combustion engine. The greater flexibility of full hybrids allows the vehicle to spend more time operating its engine only when it is at its most efficient. At low speeds and at launch, the electric motor and battery powers the car and at high speeds the engine takes over.

Series Drivetrain
This is the simplest hybrid configuration. In a series hybrid, the electric motor is is the only means of providing power to get your wheels turning. The motor receives electric power from either the battery pack or from a generator run by a gasoline engine. A computer determines how much of the power comes from the battery or the engine/generator set. Both the engine/generator and regenerative braking recharge the battery pack. The engine is typically smaller in a series drivetrain because it only has to meet average driving power demands; the battery pack is generally more powerful than the one in parallel hybrids (see below) in order to provide remaining peak driving power needs. This larger battery and motor, along with the generator, add to the cost, making series hybrids more expensive than parallel hybrids.

While the engine in a conventional vehicle is forced to operate inefficiently in order to satisfy varying power demands of stop-and-go driving, series hybrids perform at their best in such conditions. This is because the gasoline engine in a series hybrid is not coupled to the wheels. This means the engine is no longer subject to the widely varying power demands experienced in stop-and-go driving and can instead operate in a narrow power range at near optimum efficiency. This also eliminates the need for a complicated multi-speed transmission and clutch. Because series drivetrains perform best in stop-and-go driving they are primarly being considered for buses and other urban work vehicles.

Parallel Drivetrain
Some up-and-coming hybrid models use a second electric motor to drive the rear wheels, providing electronic all-wheel drive that can improve handling and driving in bad weather conditions.With a parallel hybrid electric vehicle, both the engine and the electric motor generate the power that drives the wheels. The addition of computer controls and and a transmission allow these components to work together. This is the technology in the Insight, Civic, and Accord hybrids from Honda. Honda calls it their Integrated Motor Assist (IMA) technology. Parallel hybrids can use a smaller battery pack and therefore rely mainly on regenerative braking to keep it recharged. However, when power demands are low, parallel hybrids also utilize the drive motor as a generator for supplemental recharging, much like an alternator in conventional cars.

Since, the engine is connected directly to the wheels in this setup, it eliminates the inefficiency of converting mechanical power to electricity and back, which makes these hybrids quite efficient on the highway. Yet the same direct connection between the engine and the wheels that increases highway efficiency compared to a series hybrid does reduce, but not eliminate, the city driving efficiency benefits (i.e. the engine operates inefficiently in stop-and-go driving because it is forced to meet the associated widely varying power demands).

Series/Parallel Drivetrains
This drivetrain merges the advantages and complications of the parallel and series drivetrains. By combining the two designs, the engine can both drive the wheels directly (as in the parallel drivetrain) and be effectively disconnected from the wheels so that only the electric motor powers the wheels (as in the series drivetrain). The Toyota Prius has made this concept a popular, and a similar technology is also in the new Ford Escape Hybrid. As a result of this dual drivetrain, the engine operates at near optimum efficiency more often. At lower speeds it operates more as a series vehicle, while at high speeds, where the series drivetrain is less efficient, the engine takes over and energy loss is minimized. This system incurs higher costs than a pure parallel hybrid since it needs a generator, a larger battery pack, and more computing power to control the dual system. However, the series/parallel drivetrain has the potential to perform better than either of the systems alone.