Bitcoin is the first and most widely used cryptocurrency. It is decentralized, meaning it is not controlled by any central bank or government. Cryptocurrencies are digital or virtual currencies that use cryptography for security and are based on a distributed ledger system. They are not backed by any physical asset, such as gold or silver, but their value is determined by their market popularity and the amount of trust people have in them. Cryptocurrencies are not issued by any government or central bank, and are traded on various exchanges. They are popular for their low transaction costs and ability to facilitate international payments.