The original version of roulette was invented in Europe, hence it is called European roulette. This is the most common version of roulette. However a newer type of roulette was invented in the
United States, this type is called American roulette.
In European roulette the wheel consists of 36 numbers and the zero. This means that there is an equal amount of odd and even numbers, and red and black zones on the roulette. The zero has a green colour, and gives the house a little bit of edge, since the chances of the ball falling on either red or black is slightly lower than 50%.
In American roulette, the wheel has an extra green slot with double zeros. This lowers the chances of the ball falling on either black or red even more, and gives the casinos some extra profit. People prefer European roulette, since their expected value is higher than it is in the American roulette.