> For the complete documentation index, see [llms.txt](https://cll-devrel.gitbook.io/bootcamp-2024/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cll-devrel.gitbook.io/bootcamp-2024/1.-blockchain-and-wallet-fundamentals/blockchain-introduction.md).

# Blockchain Introduction

### Blockchain

> A blockchain is a [distributed ledger](https://en.wikipedia.org/wiki/Distributed_ledger) with growing lists of [records](https://en.wikipedia.org/wiki/Record_\(computer_science\)) (*blocks*) that are securely linked together via [cryptographic hashes](https://en.wikipedia.org/wiki/Cryptographic_hash_function). Each block contains a cryptographic hash of the previous block, a [timestamp](https://en.wikipedia.org/wiki/Trusted_timestamping), and transaction data (generally represented as a [Merkle tree](https://en.wikipedia.org/wiki/Merkle_tree), where [data nodes](https://en.wikipedia.org/wiki/Node_\(computer_science\)) are represented by leaves). Since each block contains information about the previous block, they effectively form a *chain* (compare [linked list](https://en.wikipedia.org/wiki/Linked_list) data structure), with each additional block linking to the ones before it. Consequently, blockchain transactions are irreversible in that, once they are recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks.

{% embed url="<https://en.wikipedia.org/wiki/Blockchain>" %}
Wikipedia Blockchain Definition
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**Blocks** are data bundles where data is permanently recorded on a blockchain.

A block records:

* Transactions
* Timestamp
* Hash of the previous block
* Nonce

### Bitcoin (BTC)

*Bitcoin* is an innovative payment network and a new kind of money.

{% embed url="<https://bitcoin.org/en/>" %}
Bitcoin Project
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### Ethereum&#x20;

Ethereum is the community-run technology powering the cryptocurrency ether (ETH) and thousands of decentralized applications.

{% embed url="<https://ethereum.org/en/>" %}
Ethereum Project
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### Chainlink

Chainlink is the decentralized computing platform powering the verifiable web.

{% embed url="<https://chain.link/>" %}