It depends on the hot tub and where you are putting it. A (sturdy) wooden deck can usually easily support the weight of a hot tub, and a foundation usually is not needed. However, if the ground is not level, you will definitely need to pour a foundation or concrete base.
Not at all. Most concrete bases are used because they are cheaper than alternatives, but it is possible to get prefabricated hot tub or spa pads for grounding!