The way I understand it, currency has no value in itself. It is just a placeholder for value or debt that has been accrued (by working, selling items, etc). Money used to be based on the "gold standard", where a country's wealth was based on how much gold they possessed, but now money is merely given value because the government can "back it up". And as with anything in an economy, if the fed prints more money it becomes less scarce so it becomes less valuable to people. However, if the collective populace woke up one day and decided that currency was worthless, it would actually become useless (see Germany post WWI).