Someone who believes in several Gods. An example would be Norse Paganism and Greek Mythology.
Polytheism was popular in many countries before the rise in Christianity worldwide, starting in Europe. An exception to this would be Asia, whose relation to theism is something to debate.
The Gods are usually centered around a certain concept, such as water, war, and sex. Offerings to these gods were said to bring favor to oneself. Offerings could be things like cakes to things like people.
The Gods were said to be very powerful entities that either blessed or cursed the world.