These two things are consider money in all commercial transaction. I think the big difference between the bitcoin and real money is the monetary value. Fiat money has a fix amount and will remain the same no matter what happens unlike in bitcoin every minute the market value is change so therefore in bitcoin you have a money and investment in just one currency.
There are numerous differences between dollars and bitcoin. Dollars are fiat money which is inflationary due to the banks printing as much of it as they like at any time they want. Bitcoin is the opposite and is fixed/deflationary