That not only depends on the airline and the fare rules, as you've already been told in another reply, but it also can depend on the type of ticket. Specifically, at many airlines an award travel ticket is refunded in the original form, but many paid tickets are refunded as travel credit, except "fully refundable" fares are likely to refunded to the original form of payment. And some deeply discounted tickets at some airlines may not be refundable at all.

And then the terms of the travel credit also vary from airline to airline. For example, Southwest just recently made it not have an expiration date, but many other airlines still have expiration dates for them.