The answer is... it depends!

It depends on the fare rules for your itinerary. It depends on what booking engine you use.

A fairly typical fare rule will say "payment and ticketing must take place within 24 hours of booking". This means that a seat can be held for 24 hours without payment - after the 24 hours you either let it go, or you pay for it.

The problem is that while this is something you can do with the airline over the phone, or that your travel agent can do via their CRS, it's something that isn't always programmed into web-based booking agents available to the general public. When I book a KLM flight on the KLM web site I have to pay (and commit myself) immediately. Booking the same flight via Expedia I can hold it until midnight the following day.