Quick Rules Nutshell: Vodou is a Charisma tradition, just like standard shamans. But they do have a different set of spirits that they use. It's similar to how Shamans and Hermetics are different, just more so. They can use Guardian, Water, Man, Guidance, and Task spirits (so no, they can't summon Fire spirits; but then again, neither can Shamans).
Possession spirits: Traditions that are marked as "Possession traditions" means that the spirits that they summon have the Possession power instead of Materialization. You can check out the Possession power on page 197 of Street Grimoire, but the quick description is that they must take over something to affect the physical world. They can't just pop in and have their own body. This is both good and bad, on the one hand, it is harder for them because they have to make a test to take control (opposed by the object or person they are possessing). But it is also possible for them to take over anything you want; one of the enemy combatants for example. The spirits can possess anything, living creatures, corpses (this is one of the sources of the SR equivalent of Zombies), even mechanical devices (although Object Resistance is how they are resisted). Vodou spirits tend to prefer possessing living things, or at the very least things that are mobile so they can move around and "live it up."
If you want to know more about how Vodou works on a more RP-level, or how they might interact with their spirits and the like, let me know and I can get more into that.