Most, if not all major airlines have mileage programs. It is connected to your credit card and the more money you spend, the more miles you gain. Those "miles" if you gain a certain amount can get you a free, or discounted trip in a certain given class. In alliances, your airline's card(what airline's mileage program you are in) can also be used. There some restrictions depending on the airline, but it all varies from airline to airline.
American was the first to start a mileage program back in 1980 when they launched "AAdvantage".
It is also for the passenger, if a crew member wants to become a member, i guess he/she can sign up.
Give me a break, I created this username when I was a kid...