A blockchain is a distributed ledger maintained by a network of nodes. Each block contains transactions and a cryptographic hash of the previous block. This chain of hashes creates immutability. Change one block and the hash changes. That breaks the link to the next block. Change the next block to repair the link and it breaks the link to the block after that.
To rewrite history, you'd need to recalculate all subsequent blocks, and you'd need to do it faster than the network adds new blocks. This is computationally infeasible on established blockchains. Bitcoin and Ethereum are blockchains. They maintain records of transactions, who sent what to whom, in this distributed, immutable ledger. The innovation isn't the ledger itself.
Distributed databases existed. The innovation is the consensus mechanism that lets untrusted nodes agree on what the ledger contains without a central authority. The blockchain is transparent. Anyone can download it and verify that a particular transaction exists. It's decentralized. No single entity controls it. It's immutable. Past transactions can't be changed.
These properties make blockchains useful for applications where you need a shared source of truth among parties that don't trust each other. Applications beyond currency include smart contracts, identity verification, supply chain tracking, property rights, and voting.