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A bearer asset on a public ledger. You can hold it and send it. The tape does not close. That is the trick, and the risk.
A cryptocurrency is a digital asset recorded on a blockchain. Control of the asset usually means control of a private key. There is no central bank behind Bitcoin or most other coins.
You buy and sell listed spot names. Coins sit in the venue wallet until you withdraw. Prices move. After the first deposit, you can lose money. Read the risk disclosures.