Who are the people featured on U.S. dollar bills?

All this time, I thought that the figures printed on the bills were the President of the United States, but when I think about it again, Benjamin Franklin on the 100-dollar bill is actually not a President.
U.S. law does not require paper money to include the President; it only sets one strict condition: the person must already be deceased. Whoever is chosen is up to the Menteri Keuangan.
I just took a quick look at the people on dollar bills, just for fun.