In 1991, It was created by a group of researcher who created it so that a document can be digitally time stamped so that it can NOT be altered or backdated. Then, in 2009, it was adopted by a group of people or a person named Santoshi Nakamoto.
Simply, a Blockchain is chain of blocks that contains any form of information.
Blockchain is a method of recording data transaction in a distributed digital ledger which could be transactions, agreements, smart contracts or any form of data that needs to be stone-recorded and verified without an single authority’s control. In other words, You can imagine them as chain of blocks while each block contains some unique records in it.
Each block has its own thumbprint or hash, data payload (transactions, agreements or smart contracts etc) & ‘hash of previous block’. First block in the chain called the Genesis block which of course does not contain the hash of previous block because, of course, it does not exits. All following blocks are glued together with its previous’s block’s hash or thumbprint token. Each block could have a fixed size or variable size for transaction data depending on how the architecture of the blockchain has been designed. For example, Bitcoin has fixed block size while Ethereum has dynamic size for its blocks depending on number of transactions in fixed window time.
Blocks of the blockchain is incredibly difficult to temper because of various reasons. First each block contain the hash reference of previous block. Secondly, a mechanism of slowing down the creation of new blocks called the proof of work. Third, entire blockchain is replicated across entire network of nodes & it can NOT be altered without more than 50% consensus which is incredible difficult.