Blockchain is a growing list of records, called blocks that are linked using cryptography. Each block contains a hash of the previous block, a timestamp and transaction data forming a chain of blocks that is resistant from everyone. This is because in order to change a block, you'll have to change the next block since it's using the previous block's hash and it goes on for all the next blocks. In order to generate blocks, you need computational power. As long as honest people are working on generating new blocks, the blockchain is considered secure and any attackers are outpaced. The blockchain has been described as an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way.
Blockchain was invented by Satoshi Nakamoto and its first implementation was on bitcoin. Since then, a lot of people have been inspired by that brilliant idea and created lots of projects that use it.
Why brilliant?
By storing data across its peer-to-peer network, the blockchain eliminates a number of risks that come with data being held centrally. Peer-to-peer blockchain networks lack centralized points of vulnerability that computer crackers can exploit; likewise, it has no central point of failure. This is called decentralization. It is brilliant because human, for the first time in history, created something that is not owned by anyone, but everyone has the same rights to use it.
Blockchain security methods include the use of public-key cryptography.