Lesson 1: What is Blockchain?

What is Blockchain?

Blockchain is a type of database – but it’s very different from traditional databases. At its core, a blockchain is a **distributed digital ledger** that records transactions across many computers so that the records cannot be altered retroactively without changing all subsequent blocks and gaining consensus from the network.

Each block in a blockchain contains:

• A timestamp
• Transaction data
• A cryptographic hash of the previous block

This structure forms a **chain of blocks** – hence the name *blockchain*. Since each block is linked to the one before it, tampering with one would require altering all blocks after it, which is practically impossible on large networks like Bitcoin or Ethereum.

Key Features

• Decentralization: No central authority. Everyone has a copy of the same ledger.
• Immutability: Once recorded, data cannot be changed without consensus.
• Transparency: All transactions are visible to anyone on the network.
• Security: Uses cryptographic techniques to secure data and ensure trust.

Why is Blockchain Revolutionary?

Before blockchain, digital assets could be easily copied. Blockchain solves the issue of **double spending** without needing a trusted third party. This is what makes **Bitcoin** and other cryptocurrencies possible. It also opens the door to secure applications in voting, identity, supply chain, healthcare, and beyond.

Think of blockchain as a digital notebook shared across the internet. Everyone can write in it, and once something is written, it’s locked in and time-stamped. That means no one can secretly change it without everyone else noticing.

Real-World Example

Imagine Alice sends Bob 1 Bitcoin. This transaction gets bundled with others into a block. Once verified by the network, it’s added to the blockchain. Bob can now see his 1 BTC in his wallet. The whole process is recorded transparently, and cannot be undone.

Next up: In Lesson 2, we’ll learn how blockchain actually stores this data inside blocks and how hashes keep the chain secure.