In banking, what is a floating and a fixed charge?

As we all know, or can guess, that a fixed charge is a certain agreement to keep a debt at a steady and same payment plan forever for each term of payment. But many people don't understand floating charges. They sound pretty unstable, and they are. Lets say you owe me 20 bucks, and must pay me a payment each week. With a floating charge agreement, I can charge you anywhere between a two certain amounts. I could charge 3 one week, and 5.50 the next. But with a fixed charge, I must charge you exactly the same each week. 6.75 sounds good to me. Fix it. Or move to the Bahamas - quick.