The continent of Africa has been home to hundreds of indigenous tribes speaking a wide variety of languages and believing a wide variety of different spiritual ideas. One certainly cannot speak of "African religion" as if it was a single, coherent set of beliefs. The versions of these religions as they developed in the New World became known as African Diaspora religions.
When African slaves were transported to the New World between the 16th and 19th centuries, they each brought their own personal beliefs. However, slave owners deliberately mixed slaves from a variety of different backgrounds together in order to have a slave population that could not easily communicate with itself, and thus curtail the ability to rebel.
Moreover, Christian slave owners frequently forbade the practice of pagan religions (even when they also forbade conversion to Christianity). As such, groups of slaves practiced in secret among strangers united by circumstance. Traditions from multiple tribes began to mix together. They might also adopt New World native beliefs if natives were also being used for slave labor. Finally, as slaves started being allowed to convert to Christianity (with the understanding that such a conversion would not free them from slavery), they began mixing in Christian beliefs as well, either out of actual belief or out of a need to disguise their actual practices.
Because the African Diaspora religions draw strongly from multiple distinct sources, they are also commonly identified as syncretic religions.
A diaspora is a scattering of people, generally under duress, in multiple directions. The Atlantic Slave Trade is one of the most well-known causes of a diaspora, scattering African slaves throughout North and South America. The Jewish diasporas at the hands of Babylon and the Roman Empire is another fairly familiar example.
Vodou developed primarily in Haiti and New Orleans. It posits the existence of a single god, Bondye, as well as numerous spirits known as lwa (loa). Bondye is a good but distant god, so