Blockchain is a decentralized, digitized ledger that records transaction information about a cryptocurrency in a chronological order. It is comprised of a sequence of blocks, or units of digital information stored consecutively in a public database.
Blockchain is the foundational technology for cryptocurrencies as it stores all the data and transaction records. What makes blockchains so different are their decentralized records. Instead of being stored in one central location, the blockchain is stored on the computers of every of its user.
Besides their decentralized nature, blockchains are secure because of their immutability. This is done by the unique block identifier known as a hash, which is derived from the information contained in every previous block in the blockchain. This means that, in order to falsify or modify any record on the blockchain, someone would have to change every block of the blockchain. This system ensures the security of blockchains as its data are virtually unfalsifiable.
Most blockchains such as Bitcoin and Ethereum are public. This ensures that anybody can view records of transactions conducted on the blockchain, using a tool called a block explorer.