At low speeds, especially during acceleration, the electric motor does more of the work. At higher speeds, the gasoline engine does most of the work with the electric motor providing extra boost when needed.
In addition to powering the wheels directly, the gasoline engine charges the car's batteries. The batteries are also charged during braking.
Because the batteries are constantly being used and recharged, hybrid vehicles like this can be made using current battery technology. Plug-in hybrids, which would store up a lot of energy during overnight charging, require more advanced batteries.