beginner lesson • 10 min
What Is a Blockchain?
What you will learn
By the end of this lesson, you should be able to: define a blockchain as a distributed digital ledger; explain how blocks form an ordered chain; describe the role of nodes, rules, and consensus; explain why changing an earlier block can make later blocks fail validation; and distinguish permissionless from permissioned implementations.
Evidence & citations10 sources
A blockchain is a digital ledger whose records are grouped into blocks and maintained across a distributed network. (supported)
Each block normally contains a cryptographic reference to a previous block, creating an ordered chain. (supported)
Changing data in an earlier block changes its cryptographic relationships and can make later blocks fail validation, making the ledger tamper-evident or tamper-resistant rather than absolutely immutable. (supported)
Nodes are computers or software instances that participate in a blockchain network by storing, transmitting, or verifying blockchain data, depending on the implementation and node type. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
Blockchain implementations can be permissionless, allowing broad participation, or permissioned, restricting participation to approved participants. (supported)
Blockchain in simple terms
Imagine a shared notebook whose pages are copied among many computers. New records are collected onto a page, and each new page includes a cryptographic reference to the page before it. If someone changes an older page, the references connecting the later pages no longer match. The computers also follow agreed rules for deciding which new pages belong in the shared notebook. This is the basic idea of a blockchain.
Evidence & citations7 sources
A blockchain is a digital ledger whose records are grouped into blocks and maintained across a distributed network. (supported)
Each block normally contains a cryptographic reference to a previous block, creating an ordered chain. (supported)
Changing data in an earlier block changes its cryptographic relationships and can make later blocks fail validation, making the ledger tamper-evident or tamper-resistant rather than absolutely immutable. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
The core idea
A blockchain is a digital ledger: a record of information or changes that is grouped into blocks and maintained across a distributed network. The network is made up of nodes, which are computers or software instances that may store, transmit, or verify blockchain data depending on their type and implementation. Rules and a consensus process help participants agree which transactions or other state changes are valid and should become part of the shared history.
Evidence & citations6 sources
A blockchain is a digital ledger whose records are grouped into blocks and maintained across a distributed network. (supported)
Nodes are computers or software instances that participate in a blockchain network by storing, transmitting, or verifying blockchain data, depending on the implementation and node type. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
How blocks are linked
Each block normally contains a cryptographic reference to the previous block. This reference creates an ordered chain rather than a collection of unrelated records. If information in an earlier block changes, its cryptographic relationships change as well. Later blocks may then fail validation because their references no longer match. This makes the history tamper-evident or tamper-resistant: an alteration can be detected or made difficult, but the term does not mean that the ledger is absolutely immutable.
Evidence & citations4 sources
Each block normally contains a cryptographic reference to a previous block, creating an ordered chain. (supported)
Changing data in an earlier block changes its cryptographic relationships and can make later blocks fail validation, making the ledger tamper-evident or tamper-resistant rather than absolutely immutable. (supported)
The main parts working together
A simplified process looks like this:
1. Records or proposed state changes are communicated to the network. 2. Nodes participate by storing, transmitting, or verifying data, depending on their role. 3. The blockchain's rules and consensus process help participants agree which proposed changes are valid. 4. Valid records are grouped into a block. 5. The new block refers cryptographically to the previous block, preserving the order of the shared history. 6. If an earlier block is altered, its relationships with later blocks can stop matching, making the change evident to participants or validation procedures.
The exact responsibilities of nodes and the details of consensus can differ between implementations.
Evidence & citations8 sources
Nodes are computers or software instances that participate in a blockchain network by storing, transmitting, or verifying blockchain data, depending on the implementation and node type. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
Each block normally contains a cryptographic reference to a previous block, creating an ordered chain. (supported)
Changing data in an earlier block changes its cryptographic relationships and can make later blocks fail validation, making the ledger tamper-evident or tamper-resistant rather than absolutely immutable. (supported)
A simple shared-record example
Suppose a group uses a blockchain to maintain a shared record of agreed changes. The group’s network participants follow defined rules for deciding which proposed changes are valid. Once accepted, changes are organized into blocks, and each block refers to the previous one. If a participant edits an older record in isolation, the later references may no longer match, so the alteration can be detected during validation. This example illustrates the structure of a blockchain; it does not describe a particular real-world network.
Evidence & citations7 sources
A blockchain is a digital ledger whose records are grouped into blocks and maintained across a distributed network. (supported)
Each block normally contains a cryptographic reference to a previous block, creating an ordered chain. (supported)
Changing data in an earlier block changes its cryptographic relationships and can make later blocks fail validation, making the ledger tamper-evident or tamper-resistant rather than absolutely immutable. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
Key terms
- Ledger
- A record of information or changes.
- Block
- A group of records in the blockchain’s history.
- Cryptographic reference
- A reference that links one block to the previous block and helps preserve ordering.
- Node
- A computer or software instance participating in the network by storing, transmitting, or verifying data, depending on its type.
- Consensus process
- The process and rules that help participants agree which changes are valid and belong in the shared history.
- Permissionless blockchain
- An implementation that allows broad participation.
- Permissioned blockchain
- An implementation that restricts participation to approved participants.
Evidence & citations9 sources
A blockchain is a digital ledger whose records are grouped into blocks and maintained across a distributed network. (supported)
Each block normally contains a cryptographic reference to a previous block, creating an ordered chain. (supported)
Nodes are computers or software instances that participate in a blockchain network by storing, transmitting, or verifying blockchain data, depending on the implementation and node type. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
Blockchain implementations can be permissionless, allowing broad participation, or permissioned, restricting participation to approved participants. (supported)
Common misconceptions
• “A blockchain is just one database on one computer.” Not necessarily. Its records are maintained across a distributed network. • “Every node does exactly the same job.” Not necessarily. Node responsibilities can differ; nodes may store, transmit, or verify data depending on their type and implementation. • “Blockchain data can never be changed.” This is too absolute. Cryptographic links make changes evident or difficult, but the more precise description is tamper-evident or tamper-resistant rather than absolutely immutable. • “All blockchains are open to everyone.” Not necessarily. Implementations can be permissionless or permissioned. • “Consensus means every participant independently writes whatever it wants.” No. Rules and a consensus process help participants agree which transactions or state changes are valid and should be included.
Evidence & citations9 sources
A blockchain is a digital ledger whose records are grouped into blocks and maintained across a distributed network. (supported)
Nodes are computers or software instances that participate in a blockchain network by storing, transmitting, or verifying blockchain data, depending on the implementation and node type. (supported)
Changing data in an earlier block changes its cryptographic relationships and can make later blocks fail validation, making the ledger tamper-evident or tamper-resistant rather than absolutely immutable. (supported)
Blockchain implementations can be permissionless, allowing broad participation, or permissioned, restricting participation to approved participants. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
Key takeaways
A blockchain is a distributed digital ledger organized into blocks. Cryptographic references connect the blocks into an ordered chain. Nodes participate in different ways, while rules and consensus help the network agree on valid updates. Changing an earlier block can disrupt later references, making the change evident or difficult rather than guaranteeing absolute immutability. Blockchain implementations may be permissionless or permissioned.
Evidence & citations10 sources
A blockchain is a digital ledger whose records are grouped into blocks and maintained across a distributed network. (supported)
Each block normally contains a cryptographic reference to a previous block, creating an ordered chain. (supported)
Changing data in an earlier block changes its cryptographic relationships and can make later blocks fail validation, making the ledger tamper-evident or tamper-resistant rather than absolutely immutable. (supported)
Nodes are computers or software instances that participate in a blockchain network by storing, transmitting, or verifying blockchain data, depending on the implementation and node type. (supported)
A blockchain requires rules and a consensus process for network participants to agree which transactions or state changes are valid and should be included in the shared history. (supported)
Blockchain implementations can be permissionless, allowing broad participation, or permissioned, restricting participation to approved participants. (supported)
Knowledge check
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