A blockchain is a distributed digital ledger that stores information in a sequence of connected blocks.
Each block contains a collection of transactions, together with information that links it to the previous block. This link is created using a hash, a cryptographic value generated from the block's data. Even a small change to the data produces a completely different hash, making the connection between blocks easy to verify.
Transactions are exchanges of value or data between users. Before they are included in a block, they are checked by network nodes, which are computers connected to the blockchain that maintain and verify its data.
Once a block has been accepted by the network and added to the chain, its transactions become part of the blockchain's permanent history. Because each block is linked to the previous one, changing information in an older block would also break the links with the blocks that follow it.
This structure makes blockchains tamper-resistant: their history can be verified by the network, and changing previously recorded data becomes extremely difficult.
Unlike a traditional database controlled by a single organization, a blockchain is maintained by many independent computers. This distributed structure allows the network to keep a shared record of transactions without relying on one central authority.