Blockchain is  very popular now a days.
It is a chain of block that contains information this technique was described in 1991
by a group of researchers. it is a distributed ledger that is completely open to any one.
They have interesting property once a data has been recorded inside blockchain it becomes 
very difficult to change it so how does that work well each block has a data hash and hash of previous block
the data that is stored inside the block. it stores the detail of transaction sender reciever and the amount.
hash identifies the block and all of its content it is unique just like a finger print once a block is created its hash is calculated. Hash of previous block creates a chain of block this technique makes the block secure.