The history of voodoo (or vodou, voudoun) is a long and complicated story. The term "voodoo" does not describe a religion exactly, like "Catholicism" or "Islam" does, but instead is an umbrella term for a large variety of beliefs. Most of these beliefs originated in West and Central Africa, most particularly the Dahomey tribe. Over time, these beliefs morphed, weaving many disparate parts into the tapestry that is modern-day voodoo, still practiced in many different places around the world.
The word voodoo comes from the Dahomey word for "spirit," but the beliefs are an amalgamation of those of several different tribes who shared some key core concepts: they used music and rhythm to call upon their gods, they focused on the worship of ancestors in addition to other spirits, and they held the belief that powerful spirits could possess or "ride" believers in order to interact more directly with the mortal world. Before ever reaching the western world, many of these belief systems were already hundreds of years old, strongly tied to the identities of those who held them. These tribes brought their ideas and their gods, or lo'a, with them as they were carried away to slavery in the Americas and the West Indies.
French-held Haiti in the eighteenth century is where we find the roots of modern voodoo, among the homesick and battered slaves brought to the West Indies from their homes. Here is where the history of voodoo becomes complicated, and harder to follow. It is clear that plantation owners in this time began banning the worship of their slaves' gods, and even began forcing the slaves' conversion to Catholicism. Ever focused on practicality, practitioners of voodoo saw no conflict in incorporating various Catholic saints into the already wide pantheon of voodoo lo'a.
During this time period, Haiti was fairly isolated from the rest of the western world, and in this isolation, the beliefs of the slaves were able to flourish, if in secret. This is when we start to see the incorporation of the beliefs of the natives of Haiti, as well. Voodoo practitioners in this time lived harsh, violent lives, and their spiritual beliefs were focused on results. Because of this, voodoo beliefs show very little in the way of delineation between good and evil. Revenge is seen as an understandable and acceptable motivation among believers, who rarely have qualms about seeking that which they desire. This is not to say that voodoo practitioners are dangerous or evil, simply that their morals balance differently than say, a Catholic's. This is true of all religions to some extent.
In the early 1800s, just after the Louisiana Purchase, plantation owners in the West Indies and Americas began lifting restrictions on their slaves' worship. There was also a significant migration from the West Indies up into the new Louisiana purchase, especially around New Orleans. This sudden influx, coupled with legal changes, allowed for an explosion of the practice of Voodoo in the area. This included the conversion of many New Orleans blacks, and culminated in voodooists meeting and practicing their beliefs in public. The city is the heart of modern voodoo practice, in addition to being a cultural and social melting pot, drawing people from all walks of life. Modern practice is wide-spread and varied, reaching out to practitioners straight across the world from its murky history in the depths of the African plains.