In the US, if an airline involuntarily bumps you from a flight, the US Department of Transportation has rules in place to protect you, as long as you have a confirmed reservation, checked in for your flight on time, and arrived at the departure gate on time.
If the airline arranges an alternative flight that gets you to your final destination within an hour of your original scheduled arrival time, you won’t be compensated. However, if a substitute flight for a US domestic flight will get you to your destination 1-2 hours later than your original landing time, or 1-4 hours internationally, the airline must pay you double the cost of your one-way fare, up to $675.
And if the bump delays you by more than two hours domestically, or over four hours internationally, or if the airline doesn’t make substitute arrangements, the compensation doubles, with a $1,350 ceiling.
You are entitled to demand payment on the spot. Do be aware that rules in the European Union are similar, but vary. And, note that the above only applies when you are involuntarily bumped from a flight, not for delays of any other reason.
Of course, there are some exceptions.