What is a 'food bank'?

I hear about food banks all the time in the USA. What is a food bank? Who supplies it and how much food does one get and why?

What is the food like? Do you get more if you have a bigger family, are their toiletries included?

Is this over and above some sort of payment you receive? (I am guessing you get food if you are unemployed?).

What types of items are in the food supply, do you get more food if you are say, disabled as opposed to unemployed?