Blockchain is a type of record-keeping system. Unlike a centralized database, where one entity is the source of record, a blockchain database is distributed. This structure is called a distributed ledger. Within a distributed ledger, the database is pushed to multiple nodes (i.e., machines) within the network. Certain nodes verify each transaction. These verifying nodes must reach a consensus for the transaction to be committed into the ledger and be redistributed. The consensus of transactions is a key element of blockchains. No one authority is able to take control of a blockchain. Blockchains are also the backbone of cryptocurrencies.