In my view, they all exist on the same continuum, and there's often no good reason to use these terms individually: every decision about where to place your money is a "gamble", is "speculation", is an "investment", and is "saving" all at the same time.
The only common attribute from the above terms that you stand to gain or lose value in the process of these activities. The differences are extremely important however because
gambling is the opposite of investing. A
gamble involves an independent event of pure chance where the person placing the bet has no influence on the outcome, such as a dice wager. An
investment involves a meaningful choice made by the person funding it (with their time or money), such as buying a used car.
Speculation refers to an investment made with the intent of reselling in the short term for a profit.
Saving refers to an investment made with no intent in reselling in the short term for a profit. Presumably, both speculators and savers assume the thing they are investing in will generate a profit on some future timeframe.