The blockchain is a next-generation decentralized database structure. It is often described as a world computer or a public network.
Data, and the programs which control the data, are defined within applications known as smart contracts. Actors interact with these contracts using private keys and the transactions are validated by the nodes on the network.
Every node on the network has its own copy of the data alongside the rules for changing the data (embodied in smart contracts). Consensus algorithms ensure that all the individual copies of the data stay in sync. There is no opportunity for anyone to improperly alter the data as there is no single, central instance.
By not having to trust in a centralized source the system becomes secure and trustworthy. There is no opportunity for hackers and no opportunity for centralized control.