What is the difference between express and delegated powers?

expressed is the powers that are specifically said in the constitution i.e. fed govt has the power to set up mail service. delegated is powers 'received' they just depend on the time but one instince would be the federal govt saying that gay marrige must be legal everywhere. it doesnt say in the constitution the fed govt can make that decision but sometimes times change and people expect the fed govt to act ( not just this situation, its just the first one off the top of my head)