What Is Blockchain? The Big Idea

What you’ll learn

• •A blockchain solves the 'who controls the ledger' question from Day 1 by duplicating it across thousands of independent nodes instead of one institution.
• •Transactions are bundled into blocks, and each block cryptographically references the one before it, forming a chain.
• •A hash is a unique digital fingerprint of a block's data; changing anything recalculates the hash and breaks the chain link.
• •Rewriting blockchain history would require overpowering the honest majority of the entire network at once — that's why it's called tamper-evident.
• •Blockchain is the general technology; Bitcoin, Ethereum, and thousands of other networks are specific applications built on it.
• •Consensus — how the network agrees on the truth without a central referee — is covered in full in Week 4.