Spa, hot tub, jacuzzi are commonly used interchangably when referring to the product that we all love.
I think of "hot tub" as a round tub with water but no air jets.
A spa has the molded seats and air added.
Jacuzzi is actually a name brand for a tub. Most hotels have what they call whirlpool tubs which don't heat the water once it's in the tub, only circulate it through the tub.
Maybe I'm wrong about this, but this is how I understand it to be. I guess it doesn't really matter what you call it as long as you enjoy it.
- Slappy