if so, what is that and how is resting mass different than just mass?
Ok, thank you. I was just confused I had because I had always been told they had no mass at all (because they don't interact with the Higgs fields right?) so it didn't make sense to me as to why they would have resting massThe photon mass is zero and it is not very correct to talk about a photon rest mass as they cannot be at rest. For massive particles, rest mass is what physicists normally refer to when they say "mass". The term "relativistic mass" is not used much and you will have a hard time finding a physicist who refers to it as just "mass".