Everyone always asks, "What's the difference between a spa and a hot tub?" Well, the truth is the two terms are used interchangably but here is my definition, which I think may help simplify discussions:
A spa is attached to the pool and shares filtration and a hot tub is a stand alone unit independant of any pool equipment. This distinction can be important when you discuss pool chemicals. You may not want to use the same treatments in your pool/spa that you would put in your hot tub.