A blockchain has its distributed ledger nature, each node in the network has to be able to find the same end result given the same input. That means if we were to replay every transaction, we would end up in the correct state.
The crucial aspect about the blockchain is that smart-contracts can be deployed to that blockchain and that users of that blockchain can execute the code. Smart contracts executed against the blockchain are like the controller of the MVC model.
In this context, the blockchain can be treated as a cloud provider that is owned by no one. It's run by compute power of people who want to support the ethereum network.
Blockchains are essentially very secure, distributed, and decentralized databases.