A blockchain is a decentralized public ledger that records transactions on blocks that cannot be changed (immutable). Blockchain as a public ledger means it is not run by a centralized body but a copy of the ledger is stored on decentralized nodes within the network. As a public ledger, it can be viewed by anyone, making transactions transparent without the fear of manipulation.
Blockchain is the underlying technology for cryptocurrencies, which has become the money for the blockchain. Transferring cryptocurrencies and digital assets requires a certain amount of block confirmations as part of the approval process. However, due to the transparent nature of blockchains, all transactions can be monitored while remaining anonymous.
Blockchains are safe and immutable because each block is linked, meaning each block relies on the block before it to be verified. The Bitcoin blockchain was the first to be developed in 2009, but now there are a plethora of blockchains that have been developed with various attributes.