Vertical spreads come in two flavors. A debit spread costs money to open (net debit); a credit spread pays you to open it (net credit). Both cap risk and reward, but they profit from opposite forces (vertical spreads).

Debit spreads: pay for a move

You buy a debit spread expecting a directional move — a bull call spread for up, a bear put spread for down. You pay a premium, and you profit if the underlying moves your way by expiration. Time decay works against you, so you need the move to happen (theta decay). Max loss is what you paid; max gain is the spread width minus that.

Credit spreads: get paid to wait

You sell a credit spread collecting premium, betting the underlying won't move past your short strike — a bull put spread if you think it holds up, a bear call spread if you think it stalls. Time decay works for you; you profit if price stays out of the way (long vs short premium). Max gain is the credit; max loss is the width minus the credit — so credit spreads risk more to make less, in exchange for a higher probability.

Debit spreads pay you to be right about direction. Credit spreads pay you to be right about where price won't go — and let time do the work.

Choosing

Want a defined directional bet with time against you? Debit. Want to lean on theta and probability with time on your side? Credit (the iron condor combines two credit spreads). Both are defined-risk, unlike selling naked options (naked-options risk). See also how IV affects both.