Although we recommend that you never fly without your ID domestically you still might be able to board a flight. So if you are at the airport for a domestic flight without your license, the first thing you should do is to check in with your airline using your phone; and then head to the security line, the earlier the better.
In the security line, you will have to ask the TSA for a release form that states your name, the last four digits of your Social Security Number, and your home address. At this point, the TSA will be checking your Social Security records. Once the form is signed, TSA will ask you a series of identity-verifying questions. The questions vary and range from what is your mother's maiden name to where you were born. The answers are relayed to a TSA agent that verifies the answers to these questions and those you answered on the form. Most of these answers are available on your birth certificate if you are a natural born citizen.
Once your answers have been verified to the satisfaction of the TSA agent, you will be able to go through the checkpoint where there will be additional screening.
Never forget your ID when traveling internationally. The TSA agent can screen you and let you through the security checkpoint, however, the airline at the gate will not allow you to board an international flight without a passport. If you lose a passport when traveling internationally you will have to contact the nearest embassy or consulate. To be on the safe side, it is always helpful to keep of passport photo of you in a secure spot to help expedite the process of getting another passport when traveling internationally.