It's a doll that is supposed to look like a specific person. And supposedly, when you do something with the doll (say poke it in the back with a pin) the person will feel a sharp poke in the back with a pin. Some say that you can control a person's movements with a voodoo doll as well
According to voodoo rites and rituals a voodoo doll is used to inflict pain and/or punishment on someone. Whatever is done to the doll is supposed to be felt by the person the doll is meant to represent. Poke a pin into the doll, for example, and the person is supposed to experience a corresponding sharp pain.
Can be made of wood, wax, clay, or just a stuffed doll. Vaguely resembles the victim, The key part is something that belongs to the victim, preferably a piece of skin, a hair, or nail clipping. Then you have to go through a specific voodoo ritual, to make the magic. (and, no, I would not tell you the ritual, even if I knew it.) Then you can light it on fire, stick pins in it, etc, to inflict pain and suffering. Just remember, you screw up the incantation, it will come back on you.