I became interested in voodoo when I played research assistant (translation - driver, note taker, secretary and gopher) to one of the most eccentric professors ever to grace the hallowed halls of the University of Southern Mississippi - Dr. O (aka The Professor of Enemy Languages - He taught German and Russian.) The study of the voodoo religion as practiced in Louisiana was a passion of his and he did a great deal of research on the subject in the years I knew him. Research I was fortunate enough to have a front row seat to, which resulted in my meeting some fascinating people and seeing some amazing and quite unbelievable things.
I don't really intend to post a long, drawn-out dissertation on voodoo today. If anyone has any specific questions or wants to be pointed to some great research resources I would be happy to do so. Just zip me an e-mail and I'll do the best I can to help. (firstname.lastname@example.org)
I will tell you most of the movies with voodoo themes have picked bits and pieces from various versions of voodoo or hoodoo and therefore don't give a really great picture of the practices of specific versions of the voodoo faith. Yes, it is a faith, a religion and its roots can be found in African spiritual practices, Catholicism, and rituals from many Caribbean island beliefs. A priest is called a oungan, a priestess a mambo. Spirits are lwa or lao and hold a similar position as saints in Catholicism. Catholic saints can be called upon as well. Gris-gris can be worked for good or evil and there is always a price to pay.