What is the difference between a fiat currency and a gold-backed currency?

What is the difference between a fiat currency and a gold-backed currency?
Fiat currency is based on faith that it is worth something, much like an IOU. If the authority printing that currency has good credit, the currency will be worth more, and the opposite is true, as well. Gold-backed currency is just that: currency which represents the exact value of gold printed on it. So, a $1 treasury note = $1 in gold. Traditionally, with a gold-backed currency, you, the holder of that note, would be able to go down to your local bank and exchange that note for the same amount of gold. In short, with a fiat currency, everyone agrees that it's worth that amount, and it is subject to national credit ratings. With a metal-standard currency, the currency is based on the price and value of a particular metal, like gold.

There are currently no gold-backed currencies. Every currency in the world is a fiat currency.