As name suggest it is a block of chain. A bloc contains information, it could be anything, most popular example of blockchain is Bitcoin now these days.
This technique originally decribed by a group of researchers in 1991, The first blockchain was conceptualized by a person (or group of people) known as Satoshi Nakamoto in 2008.
A blockchain is distributed ledger that is completely open to anyone. Once data has been recorded inside blockchain it would be very difficult to change it.
How it works?
Each block contains
3) Hash of previous block
Data - The data that is defined or stored into block it depends on their types
For example Bitcoin has From, To, and Amount property in data.
Hash is unique long string we can compaire it as our fingureprint. If we are going to change something inside the block it will also generate new hash for that block.
Hash of previous block
Hash of previous block creates a chain, and it contains the previous block hash
If any block which does not have previous block hash called Genesis block, It will be always very first block of blockchain.