As mentioned, blockchain is the underlying technology of bitcoin. Blockchain is a public distributed ledger in which transactions are recorded in chronological order. Any record or transaction added to the blockchain cannot be modified or altered, meaning transactions are safe from hacking. A block is the smallest unit of a blockchain, and it is a container that holds all the transaction details. A block has four fields, or primary attributes:

1. Previous hash: This attribute stores the value of the hash of the previous block, and that's how the blocks are linked to one another.
2. Data: This is the aggregated set of transactions included in this block—the set of transactions that were mined and validated and included in the block.
3. Nonce: In a “proof of work” consensus algorithm, which bitcoin uses, the nonce is a random value used to vary the output of the hash value. Every block is supposed to generate a hash value, and the nonce is the parameter that is used to generate that hash value. The proof of work is the process of transaction verification done in blockchain.
4. Hash: This is the value obtained by passing the previous hash value, the data and the nonce through the SHA-256 algorithm; it is the digital signature of the block.

SHA-256 is a cryptographic hash algorithm that produces a unique 256-bit alphanumeric hash value for any given input, and that is the unique feature of this cryptographic algorithm: Whatever input you give, it will always produce a 256-bit hash.

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