Blockchain Fundamentals
What Is a Blockchain?
What a blockchain is, how it reaches agreement, and where it actually helps.
In this lesson
- How blocks are chained together over time
- What 'decentralized' really means in practice
Key takeaways
- 1Each block references the previous one, forming a tamper-evident chain
- 2Many participants keep identical copies
- 3Changing history would mean rewriting every copy at once
Lesson summary
A blockchain is a timeline.
Mental model
The core idea behind a blockchain
A blockchain is a timeline. Each new block carries a reference to the prior block, so the record becomes a linked chain of evidence rather than a loose folder of entries.
- How blocks are chained together over time
- What 'decentralized' really means in practice
Mechanics
How to reason about a blockchain
A block contains transactions plus metadata about the previous block.
Changing an old transaction changes the block fingerprint and breaks the link to later blocks.
Nodes reject histories that do not satisfy the network's rules.
- Each block references the previous one, forming a tamper-evident chain
- Many participants keep identical copies
- Changing history would mean rewriting every copy at once
Example
A concrete a blockchain example
If someone changes a payment from last week, every later block would no longer match the old chain. The attacker would need to rebuild and convince the network to accept that version.
Common mistakes
The usual a blockchain trap
Decentralized does not mean every user controls the network equally. It means no single party should be able to force a private version of history on everyone else.
Risk notes
What can go wrong with a blockchain
Weak node diversity, centralized infrastructure, or a small validator set can reduce the practical decentralization that the chain claims to have.
- Describe how blocks link together.
- Explain why old edits are visible.
- Identify one centralization risk.
Practice
Put a blockchain to work
- Describe how blocks link together.
- Explain why old edits are visible.
- Identify one centralization risk.
Review
Key terms
- Block
- A batch of transactions bundled together and cryptographically linked to the previous block.
- Blockchain
- A shared, append-only ledger replicated across many computers, secured by cryptography and consensus.
- Node
- A computer that stores and validates a blockchain's ledger.
- Validator
- A staker who proposes and attests to blocks in Proof of Stake.
- Large Language Model (LLM)
- An AI model trained to predict and generate text from context.
Source notes
Editorial references
These references are starting points for verifying the mechanisms, risk checks, and product context behind this lesson.
Before you continue
Can you do these?
- Describe how blocks link together.
- Explain why old edits are visible.
- Identify one centralization risk.
Related learning
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Blocks are added to a blockchain…