A cryptocurrency is a digital asset whose issuance and transfers are recorded on a blockchain, a distributed database verified by a network of computers rather than by a central entity like a bank.
Bitcoin, Ethereum, and the rest
Bitcoin was the first and remains the largest by market capitalization, designed mainly as a store of value. Ethereum, besides being a cryptocurrency, is a platform that lets you run smart contracts, the foundation of most DeFi applications.
Volatility and risk
Cryptocurrencies are, in general, far more volatile than traditional financial assets, and they don't carry the same guarantees as a bank deposit or a regulated fund: there's no Deposit Guarantee Fund or securities regulator backing their value.
Taxation in Spain
Gains from buying and selling cryptocurrencies are taxed as capital gains within the savings base of IRPF (Spain's personal income tax), and since 2024 there's an obligation to declare balances held abroad (Form 721).