When we talk about traditional currencies (post-Nixon-shock), we refer to fiat currency. Fiat currency is issued by governments and isn’t backed by any commodity (like gold). That means its value is based on the legitimacy of the government issuing the currency.
Examples of fiat currency include the US Dollar, Euro, Chinese yuan, Japanese yen, etc.
Bitcoin, on the other hand, is a currency that derives value from 1) its utility as a medium of exchange (technology) and 2) price speculation (as an asset). Unless Bitcoin is universally adopted, these factors are inevitably tied together.
If Bitcoin has no utility, it has no perceived value. If it has no perceived value, it has less utility.
The other main difference between fiat currency and Bitcoin is that Bitcoin is entirely digital. There is no paper version of it. However, just about every major government in the world has created its own digital currency tied to paper fiat. But that’s for another time.