Standby used to be dramatically different. In the 1990s, passengers could buy cheap standby tickets instead of full-fare tickets. If the flight wasn't full, they could use their standby ticket and take one of the empty seats.
This practice was ended when airplanes were unable to properly fuel for flights due to not knowing the amount of people on the plane until the last minute. Airlines also recognized that they were losing a large number of full-fare tickets due to standby.
As of 2010, most airlines only offer standby flights in special circumstances. One of these is if you miss your flight. In this situation, some airlines will offer you a standby seat on another flight. This is not really saving money, as you already paid for a ticket, but it does save you the price of buying a new ticket.
Another situation is if you arrive to the airport early. If this happens, the airline may put you on standby for a flight so you don't have to wait as long.
Finally, some airlines charge extra fees (typically between $25 and $75) for standby flights. Southwest, for example, only extends standby privileges to full-fare passengers; if you are not a full-fare passenger (and most are not), you would have to pay the difference to get on a standby flight. Rules vary by airline, but usually you have to fly on the same day and to the same destination if you use standby.
