Block

A Block is a collection of data and a unique alpha-numeric value generated using the data called Hash. It also contains Hash of previous block.

Data – the data inside a block is dependent on the application, it could be anything based of the use case.

Hash – it's a code generated based of the data in the block. It is always unique even though the data could still be same.

Previous Hash – it's the hash code of the previous Block. This is how a chain of blocks are created in a Blockchain.

Each Block contains Hash of the previous Block to make it a Chain of blocks. This ‘previous hash’ helpsto navigate the entire chain and makes it hard to tamper data of any block.

The first block in the Blockchain won’t have a Previous Hash populated and it’s called a Genesis Block.

Immutable

Once a block is written, it’s immutable, meaning it cannot be altered. If a block is altered by any malicious actor, it’s Hash chances which is already recorded in the next chained block.

For instance, if a malicious actor tries to make a change to the data on Block #2, it’s corresponding Hash value needs to be regenerated. If a Hash value is regenerated for Block #2, the Previous Hash in Block #3 gets delinked, hence breaking the Chain.

So theoretically if someone changes a block, the entire chain must be changed which requires heavy processing power.

With today’s ever-growing capacity of processing power, it could theoretically still be possible to change the entire chain. To avoid that, any change to the Block must go through something called Consensus model.

So Blockchain is a collection of records linked to each other, strongly resistant to alterations, protected using cryptography with a foundation of distributed processing and persistence.