A blockchain is a growing list of database records linked to previous blocks of database records using cryptographic techniques. These blocks can be monitored publicly and can be updated on the network using distributed ledger technique. Updating or identifying these new blocks is known as Mining which uses Proof of Work aka Nakamoto consensus mechanism. The node which identifies the block is rewarded with a virtual coin (cryptocurrency) having the smallest unit as Satoshi.
By its architecture, a blockchain is resistant to modification or alteration of its data. This is because once the data is recorded in any given block it cannot be altered retroactively without the alteration of all previous blocks. For using it as a distributed ledger, a blockchain is typically managed by a peer-to-peer network collectively adhering to a protocol for inter-node communication and validating new blocks. Although data in the blockchain is not unalterable, blockchains are considered secure by design and symbolize a distributed computing system. The blockchain has been described as "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way".
Blockchain data is Immutable signifying that no data tampering is possible within the network once added to the blockchain. Looking at it in another way, blockchain only supports two operations, Read and Write.
Read Operations: To retrieve or read existing data from the blockchain network
Write Operations: To add information and data to the blockchain network during mining
Another key property Transparency that blockchain offers is how anyone with the right tool can verify the data once written into the public blockchain. Transparency among the blocks ensures that the public can trust the network.
Satoshi Nakamoto was the first-ever group/person who has implemented the blockchain concept in Bitcoin Blockchain in 2008. The cryptocurrency which is released as a reward to the Miners and transaction validators is known as Bitcoin (BTC). Till 2012, the reward was 50 BTC for every single mined block which gradually decreased with time while the difficulty to identify the new block increased exponentially.
The first block (Genesis Block) of bitcoin was hardcoded with all the necessary conditions (reward, timestamp, transaction hash record, previous block address, difficulty, 21 million quantity, etc) to run the blockchain on a distributed network.
During the early days of its implementation, bitcoin was known only as a cryptocurrency aka virtual currency which had a high fluctuating price associated with it until people started to learn the blockchain technology behind bitcoin.
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