You might be thinking of an ID90 type thing. Different airlines have agreements with others that allow you to get discounted tickets (up to 90% off normal price). It is different for every airline who they partner with. Some airlines (like Delta) have other deals for families of pilots to fly their airline. Depends on the airline.

I my family hasen't used any of these yet, so I can't go into more helpful details. I believe that they are space available, but I am sure someone will correct me if I am wrong.