Let's see:

A crew is the first "gang" you join. You do jobs, you make your bones. If your good and pure Italian, the Family may open their books put you up for your button.

A button is/was a made member of the family. In the 1940's, it was easier to be made than in recent years (more suspcion now for obvious reasons). Made members were the soldiers. Remember when Sonny told Tom that 100 button men were on the streets 24/7 looking for Sollozzo. If you're a good soldier, you get promoted just like in the military. Eventually, hard work, dedication, earning potential and a lot of blood could earn you your own regime, making you a lieutenant or Capo.

A Capo ran his own regime of buttons and crews. They reported to the underboss and/or consigliere. Often times, the head capo got the title underboss, if for no other reason by default only. Don's paid more attention to their consigliere than their underboss (Fredo vs. Tom to Mike)

So, boys and girls, that's my Schoolhouse Rock version of "I'm just a bill", cosa nostra style. orange

"The Godfather was a man to whom everybody came for help and never were they disappointed. He made no empty promises, nor the craven excuse that his hands were tied by more powerful forces in the world than himself. It was not necessary that he be your friend, it was not even important that you had no means with which to repay him. Only one thing was required. That you, yourself, proclaim your friendship. And then, no matter how poor or powerless the supplicant, the Godfather would take that person's troubles to his heart. He would let nothing stand in the way to a solution of that person's woe. His reward? Friendship, the respectful title 'Don' and sometimes the more affectionate salutation of 'Godfather.' Perhaps, to show respect only, never for profit, some humble gift - a gallon or homemade wine or a basket of fresh baked goods on a holiday. It was understood to proclaim that you were in his debt and that he had the right to call upon you at any time to redeem the debt by some small service." -- Mario Puzo, The Godfather (1969).