It's a long story.

1)So, fiat currencies are backed by gold reserve.
2)Fiat currencies are centralized meaning every transaction needs to be approved by the Central Bank
3)and the amount of $ is under the Central Bank control, depending on if there is inflation or not.

Cryptocurrencies as bitcoin:
1)have nothing behind them as gold
2)decentralized (but there're some centralized currencies). Transactions are done by miners so to say.
3)no one can control the amount of btc or issue more than was prescribed by the system.

I did't cover all the differences, but you can make your own research via Cointelegraph or Coindesk; watch the price change over some period at coin360 or something.
Good luck!