Simple Definition:

Voodoo is a religious practice [a set of beliefs and rituals followed by a group of people] that originated in Africa and is characterized by the belief in the power of spirits [supernatural beings] and the use of rituals [a series of actions believed to cause a magical effect when done in a specific sequence] and charms [objects believed to have magical powers]. It is often associated with Haiti, where it is practiced as a major religion.

Example: In voodoo, practitioners communicate with spirits through rituals and use charms to bring good luck or protect against evil.

Very Simple Definition:

Voodoo is a religion from Africa that believes in spirits [magical beings] and uses special rituals and objects.

Example: People who practice voodoo believe that spirits can help them and they perform ceremonies and use special items like charms to communicate with them.