Hoodoo is indeed an offshoot of Voodoo and the term Hoodoo itself is said to have come about in the 1800s, no conincidence about the same time that Voodoo would be developing in Haiti and the US.
But the difference is that, Voodoo is a religon and a way of life that has a complex system of spirits, and Dieites that are worshiped. Hoodoo on the other hand is the practice of the Craft of voodoo, litteraly, the spells, rituals, magick. etc...without putting any spiritual meaning to it.
With Hoodoo it is merely the act you do, but there is no worship behind it. It is just the pure magick of it.
People that practice Hoodoo are not of the Voodoo beleif system but they borrow and use the same techniques that voodoo implies.