I found this question on Quora.
Here’s my answer:
The word gambling is usually reserved for things with unknown outcomes where the actor has no ability to affect the outcome.
Gambling is different from games where the outcome is determined by the actor’s skill. Roulette is gambling. Poker is a game of skill.
Because athletes can improve their own performance in order to succeed, athletics is a game of skill. Betting on athletics is gambling because the bettor has no power to affect the outcome.
Different legal districts will sometimes categorize the same games differently though. Some games are hard to evaluate.