A blockchain (chain of blocks) is a database shared across thousands of computers (nodes) around the world, where transactions are grouped into blocks that are cryptographically linked to one another in chronological order.
Why it's hard to tamper with
Each block includes a cryptographic summary (hash) of the previous block. If someone tried to modify an old transaction, they would have to recompute that block and every block after it, while also convincing the majority of the network to accept that alternative version — in practice unfeasible on large networks like Bitcoin or Ethereum.
No central authority
Unlike a bank's database, no single entity controls or can unilaterally censor the blockchain: validation rules are run independently by each node, and consensus is reached through mechanisms such as proof of work (used when mining) or proof of stake (staking).
Not every blockchain is a cryptocurrency
Bitcoin and Ethereum are examples of public blockchains with their own native cryptocurrency, but the technology is also used in private networks for supply-chain traceability, notarial records, or other uses that don't necessarily involve a tradeable token.