These two terms show up constantly in the same conversations, and most explanations online use them as if they mean the same thing. They do not, and the difference actually matters once you are choosing what to spend your time on.
What arbitrage betting is
Arbitrage betting means finding a situation where two different sportsbooks disagree enough on the price of the same outcome that betting both sides, with your own real money on each side, locks in a small profit regardless of the result. No promotion is involved. The profit comes purely from a temporary pricing gap between two books, and it exists for as long as that gap exists, which is often minutes, since books adjust prices toward each other quickly once enough people bet into the gap.