Have you ever tried to explain blockchain to your friends or colleagues but you don't have an app where you can easily visualize it?
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The blockchain is a set of multiple transaction ledgers kept in sync. This implies the use of a distributed database.
Each transaction contains a unique hash of its information and it is linked to the previous transaction creating a chain. Once a transaction is inserted in the ledger and there is a consensus in the ledgers, it is final, since to change it, you would have to update all the subsequent transactions in all ledgers.
Some blockchains (more common in cryptocurrencies) have the concept of proof of work, meaning each transaction hash must comply with a rule. This rule check is used to confirm the transaction in the multiple ledgers. For a peer to validate the block it must try different NONCE's while hashing the block until it gets a hash that complies with the rule. This also serves the purpose of regulating how difficult it is to validate a block.