There are actually two voodoo musuems in the French Quarter, and entrance to one gives you free entry to the other.
We went to the voodoo museum on Dumaine Street first. It's not very big at all--only a couple of rooms and a corridor and jam-packed with dolls' pictures, altars, and offerings of the voodoo religion, including Blanche, a huge white python used in the ceremonies. There are a few information boards around that detail aspects of voodoo, but no coherent explanation. In a way, this makes the displays of strange jumbles of religious symbols and gaudy trinkets even more compelling.
The voodoo museum off Decatur Street is more modern. In the airy room downstairs, there's a video about contemporary voodoo practice (and comfortable sofas). Upstairs, there's another collection of voodoo paraphernalia. The displays of bones, crosses, and ribbons still aren't explained, but that seems to be intrinsically voodoo--strange juxtapositions that combine and produce some sort of power.