What this lesson solves

Before going deeper into Web3, it is worth answering one basic question: why use a blockchain instead of a normal database? Once decentralization, consensus, immutability, and public verifiability are separated clearly, later topics such as wallets, contracts, DeFi, and agents become much easier to reason about.

A practical definition

A blockchain can be understood as a shared ledger system maintained by multiple parties, updated under explicit rules, and verifiable by outsiders.

That definition contains four important ideas:

• multiple parties maintain the system
• records are appended under rules instead of rewritten at will
• state changes require consensus
• external observers can verify the result

What blocks and chains mean

Block

A block is a batch of data grouped together for a period of time. It usually contains:

• a list of transactions
• a timestamp
• a reference to the previous block
• information required by the consensus process

Chain

The chain is the ordered connection between blocks. As more blocks are added, changing past history becomes increasingly difficult.

Why it differs from a normal database

A traditional database usually has a clear administrator who can modify or rewrite data directly.
A blockchain is not absolutely unchangeable, but changes are much more constrained and expensive because they require broader network coordination instead of a single operator.

Its value is not speed. Its value is:

• multi-party coordination
• asset settlement
• verifiable shared state
• reduced dependence on one trusted operator

State, transactions, and block explorers

State

State is the current global result of the chain, for example:

• an address balance
• a contract variable
• the size of a protocol pool

Transaction

A transaction is an input that requests a state change.

Block explorer

A block explorer is the easiest way to inspect chain activity. It does not define truth, but it helps you check:

• whether a transaction succeeded
• what an address has done
• where a contract was deployed

What decentralization actually means

Decentralization is not one single property. It usually involves at least three layers:

Infrastructure layer

Nodes are distributed across different operators.

Permission layer

State changes cannot be decided unilaterally by one administrator.

Economic layer

The system has incentives and constraints rather than relying entirely on one company to fund and operate it.

Problems blockchains are good at

Blockchains are more suitable when:

• many parties need to read and write the same state
• asset transfers must be publicly verifiable
• execution rules should not depend on one platform
• history and outputs must remain traceable

They are less suitable when:

• a single organization controls the whole system
• latency and throughput are the highest priority
• the data should stay private
• no settlement or public verification is needed

An AI × Web3 example

If an AI agent only reads data locally and produces suggestions, blockchain may be unnecessary.
But if it needs to:

• manage onchain assets
• trigger publicly verifiable execution
• interact with multiple protocols
• prove what actions it performed

then blockchain starts to matter because it provides the trusted execution and state record layer.

Minimum takeaway

After this lesson, you should be able to explain:

• the key difference between a blockchain and a normal database
• what blocks, chains, states, and transactions are
• why decentralization is a multi-layer property
• what kinds of problems are worth putting onchain

What comes next

The next lesson moves into smart contracts and shifts the question from “who maintains the ledger” to “how rules are executed automatically.”

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