Short answer …

Strictly speaking, ‘Jacuzzi’ is a brand name for a tub of hot water using jets of air to produce bubbles and a massaging effect, made by the Jacuzzi company.

A ‘hot tub’ is any tub with hot water – either with jets or without. However, ‘jacuzzi’, ‘hot tub’ and ‘spa’ are often used interchangeably.

For more details …

Click here or here (includes hot tub, Jacuzzi, spa and whirlpool)

or see the results of a web search.