The blockchain is a public immutable decentralized global ledger powered by Bitcoin.
In the blockchain, transactions are part of blocks. Each block refers to a previous block adding to previous proofs-of-work, which forms a chain of blocks. Once a chain is formed, it confirms all previous bitcoin transactions and secures the network. Because of the proof-of-work and the blockchain, this has created an immutable open and decentralized ledger. This not only helps solve the double-spending problem, but it opens the doors for a myriad of powerful applications.
There hasn’t been a time in human history where there was a decentralized global ledger that couldn’t be changed. Things such as smart contracts and proof-of-existence are now possible, thanks to the Bitcoin network and its public ledger. With this technology, history cannot be manipulated or rewritten.
In January 2009, the very first block of the blockchain occurred, the Genesis Block, and the rest as they say, is now history.
Bitcoin solves the Byzantine Generals Problem through the use of proof-of-work and the blockchain. Satoshi Nakamoto said in an email exchange in 2008, “the proof-of-work chain is how all the synchronization, distributed database and global view problems you’ve asked about are solved,” when referring to the Byzantine Generals Problem.