Blockchain is a type of shared database that stores data in blocks and these blocks are linked together through cryptography. It stores data electronically in digital format. Blockchains are majorly known because of their role in cryptocurrency systems e.g. Bitcoin  for maintaining a secure and decentralized record of transactions.

Blockchain was first outlined by Stuart Haber and W. Scott Stornetta, two researchers who wanted to implement a system where document timestamps could not be tampered with, in 1991. But it was actually implemented in 2009, an anonymous developer named Satoshi Nakamoto created bitcoin using Blockchain and that's when cryptocurrency boomed.

Each block contains some record and this record is connected through a chain. This chain is decentralized which means the access is shared not just with one person, but with entire authority, so it is very difficult to modify the records in the block which makes it reliable.New blocks are always stored linearly and chronologically ,that is, they are always added to the “end” of the blockchain.

Decentralized blockchains are immutable, which means that the data entered is irreversible. For Bitcoin, this means that transactions are permanently recorded and viewable to anyone.

Each block contains Relevant Information/Data (In Bitcoin’s case the relevant information can be from where this is coming and where it will go), Hash (Every block has a unique key/code which is called hash.) Previous block hash. Which helps it in achieving decentralized security and trust in several ways.

• Since every block contains previous block hash, it becomes easy to track the history of the blocks.

• The very first block which does not contain the previous block hash is called Genesis Block, which means the very first block.

• Decentralized blockchains are immutable, which means that the data entered is irreversible. For Bitcoin, this means that transactions are permanently recorded and viewable to anyone.

Why is it near to impossible to modify data in blockchain?

 

If we want to change information in any of the blocks, then the hash of that block will be corrupted and then the next block containing the previous block hash will be corrupted and this process will go on till the last block. So all blocks will be corrupted from where you changed the data.Now if you say I can change the hash of all the blocks then that will be near to impossible because it will take a lot of time. If we consider Bitcoin as an example, then in Bitcoin each block takes nearly 10 minutes to change the hash. If there are one crore blocks then it will take forever to change the hash of all the blocks.

Also, one copy of this block chain is always shared with authority people in that network,so even if you change in one copy, the data will still be there in all other copies.

 

Then how can we change/add data?

So if you have to change any data block , you need to create that block and  share that block with everyone in the authority.Then there will be voting and you need to collect  50% votes in your favor that you want to add a block in the chain. If the majority agrees then that block can be added. This rule is called the consensus rule.

Uses of Blockchain:

 

1.Cryptocurrency

2. Real estate

3.Hospital Management

4. Banking and Finance