State laws differ but in general if you are operating a vehicle and have a driver's license but forgot it at home, or otherwise did not have it physically on you, you can be ticketed if pulled over and requested by an officer to produce it.Or if you are lucky and have your license number memorized the law enforcement officer may be able to look up your license and verify you have a valid driver's license this way and not ticket you.
If you are cited, it can vary by state or jurisdiction if you are cited for driving without a license or for not having a valid license in your possession.
The good news is that most areas will give you the opportunity to show up before your court date and produce a driver's license that was valid before the date and time of the original infraction so that the ticket can be dismissed. Some jurisdictions may require fines or administration fees still be paid though.
Keep in mind that this varies from jurisdiction to jurisdiction and state to state. If you were cited for not having your driver's license in your possession when you were stopped for a traffic violation, contact the court listed on the ticket and also perhaps your state's Department of Motor Vehicles to see if you can show your valid license and get the citation dismissed.