Blockchain is a distributed database existing on multiple computers at the same time. It is constantly growing as new sets of recordings, or ‘blocks’, are added to it. Each block contains a timestamp and a link to the previous block, so they actually form a chain. The database is not managed by any particular body; instead, everyone in the network gets a copy of the whole database. Old blocks are preserved forever and new blocks are added to the ledger irreversibly, making it impossible to manipulate by faking documents, transactions and other information.
All blocks are encrypted in a special way, so everyone can have access to all the information but only a user who owns a special cryptographic key is able to add a new record to a particular chain. As long as you remain the only person who knows the key, no one can manipulate your transactions. In addition, cryptography is used to guarantee synchronization of copies of the blockchain on each computer (or node) in the network.