The carousels are the circular groups of machines. There are usually three or more machines in the group. Some of the larger carousels will have an attendant placed in the middle to help with change. The machines in the carousels are usually the same denomination. It is rare to see a quarter machine in the middle of a dollar carrousel.
Banks are the long rows of machines that dominate the casino floors.
Usually the rows are back to back which allows the casino to put more machines in a smaller amount space. Its common to see one row of machines for one denomination while the machines in the row backing it in the bank is a different denomination. When I'm in the casino I like to people watch. I don't know how many times I have seen an unsuspecting player put a coin in the machine only to see it drop right through. They may repeat this two or three times before they realize that they just put a quarter in a dollar machine.