What is a bitcoin? It’s a ledger — or a database — in which each transaction corresponds to a record of the previous one and a record of the next one. So in this way, bitcoin is like a ledger with millions of records. The ledger stores transactions into an immutable, decentralized, trustless, digital ledger.
Bitcoin started out in 2009, and it was the only currency worth using but now it is used in over 140 countries and millions of accounts worldwide. It’s an alternative to fiat currencies like USD, CAD, Euro and others, because it is totally independent of national governments, and it doesn’t rely on the banks for reserves. (There are some that believe that only a few currencies like USD will ever be able to break away from the fiat system — but most currencies are just as valuable, though.)