Blockchain Fundamentals

Introduction

A blockchain is a distributed ledger: a database that is copied and synchronized across many independent computers, called nodes, instead of being stored on a single server controlled by one entity. Every participant holds a copy of the same history, and that history is agreed upon collectively rather than dictated by a central authority.

The term "blockchain" describes the structure of that ledger: data is grouped into blocks, and each block is cryptographically linked to the one before it, forming a chain. Once a block is added and confirmed by the network, altering it would require rewriting every block that came after it on every copy of the ledger at the same time: which is what makes the record practically immutable.

This lesson walks through the core mechanics behind that idea: how blocks are built, how nodes agree on which version of the ledger is correct, how the network is organized into layers, and how users interact with it safely through wallets.

How Blockchain Works

At a high level, a blockchain network performs four repeating actions: transactions are broadcast, nodes validate them, valid transactions are grouped into a block, and the network agrees on adding that block to the chain.

Anatomy of a Block

Every block is made of a header and a body. The header contains metadata used to link and verify the block; the body contains the actual transaction data.

• Previous Hash: links this block to the one before it, forming the chain.
• Merkle Root: a single hash that summarizes every transaction in the block, allowing quick verification without re-checking each one individually.
• Nonce: a number miners adjust repeatedly while searching for a valid block hash under Proof of Work.
• Block Hash: the fingerprint of the entire block header, recalculated by every node to confirm the block is valid.

Hash Demo

A cryptographic hash function takes any input and returns a fixed-length string of characters. The same input always produces the same output, but even a one-character change produces a completely different result. This is what makes tampering with a block detectable.

Notice the second input only changes the capitalization of one letter, yet the resulting hash is entirely different from the first: this is called the avalanche effect. Every hash on this page is computed live in your browser using SHA-256; nothing is sent to a server.

Consensus

Because there is no central authority, the network needs a way for independent nodes to agree on a single, shared version of the ledger. The two most widely used models are Proof of Work and Proof of Stake.

Layer 1 vs Layer 2

As blockchains grew in usage, a single base network struggled to process every transaction quickly and cheaply. This led to a layered architecture.

Wallets

A wallet does not "store" coins the way a physical wallet stores cash. It stores the private keys that prove ownership of assets recorded on the blockchain.

A common practice is to keep small, frequently used amounts in a hot wallet and larger holdings in a cold wallet.

Security

Blockchain security is a mix of cryptography, economic incentives, and network design.

• Private keys are the real target. Most losses come from stolen or leaked keys, not from breaking the blockchain's cryptography.
• 51% attacks. If a single actor controls the majority of a network's mining power or stake, they could theoretically rewrite recent history.
• Smart contract risk. Code deployed on-chain can contain bugs; audits reduce but do not eliminate this risk.
• Phishing and social engineering. The most common attack vector targets the user directly, not the protocol.

First Steps

A practical path for putting this lesson into practice, from observation to a safe first interaction.

1. 1

   Explore a block explorer

   Open a public block explorer for any major chain and inspect a real block: its hash, its transactions, its timestamp.

2. 2

   Create a wallet on a testnet

   Install a wallet and switch it to a test network, where assets have no real value, to practice safely.

3. 3

   Send a test transaction

   Request free test tokens from a faucet and send a transaction to see the full lifecycle in action.

4. 4

   Continue to Module 02

   Move on to the next lesson in the Academy to go deeper into wallets and self-custody.

Glossary

Node

A computer participating in the network, holding a copy of the ledger and enforcing its rules.

Hash

A fixed-length output produced by feeding data into a cryptographic hash function.

Nonce

A number that miners vary to find a valid block hash under Proof of Work.

Merkle Root

A single hash summarizing all transactions in a block, used for quick verification.

Validator / Miner

A network participant responsible for proposing and confirming new blocks.

Private Key

A secret value that proves ownership of an address and is used to sign transactions.

Rollup

A Layer 2 solution that executes transactions off-chain and posts compressed results back to Layer 1.

Double-Spending

Attempting to spend the same digital asset more than once, prevented by network validation.

Quiz

Check your understanding of this lesson before moving on.