A block chain is a chain of blocks that consist of information. This Technique was originally describe in 1991 by a group of researchers and was originally used to timestamp document so it will be impossible to backdate them. It went unused until it was adopted by Satoshi Nakamoto in 2009 to create a digital currency, bitcoin. Block chain technology is most simply defined as a decentralized, distributed ledger that records the provenance of a digital asset. Once data has been saved in a block chain, it becomes very hard to change it. Each block contains some data, the hash of the block and the previous hash. The data inside a block depends on the type block chain. The bitcoin block chain store the sender, the receiver and the amount of bitcoins that was sent.
The block chain also have an hash, you can compare this to a finger print it identifies a block and all of its contents and it's always unique, just as a fingerprint. Once a block is created a hash is created,changing something inside the hash will change the block and that block it's never the same
The third element is the hash of the previous block. This creates a chain of blocks and make it secure.
The first block is very special, it can't point to a previous block, cause it's the first block, so it doesn't have a previous hash. This block is called a Genesis block.
Once to you change the hash of a block the other blocks will be invalid, So hashes is not enough to prevent blocks from been tampered with,that's why there's a mechanism called proof of work, this is a mechanism that slow down the creation of node blocks. This makes it very hard to tamper with, because if you tamper with one block, you will have calculate the proof of work for all following blocks. so the security of block chain comes with a hash and the proof of work mechanism, but there is one more way block chain secure itself and that's by been distributed, instead of using a central entity to manage a chain, block chain uses a peer 2 peer network (p2p) and everyone is allowed to join. When someone join this network, he gets a full copy if the block chain, the node can use this to verify that everything is still in other
When someone creates a new block, the block is sent to everyone on the network, each node, then verifies the block make sure it hasn't been tampered with and everything checks out. Each nodes add the block their own block chain all the nodes in this network create Consensus, they agree about what block launcher are valid and which aren't, Blocks that are tampered with will be rejected by other nodes in the network.
Block chain is still evolving and we will not be able to exhaust the usage of block chain technology or uncover fully, the abilities of block chain.