Blockchain is a distributed, immutable ledger that maintains a continuously growing list of records (blocks) linked and secured using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data.

Key characteristics:

• Decentralized: No central authority controls it
• Immutable: Once data is recorded, it's extremely difficult to change
• Transparent: All transactions are visible to network participants
• Consensus-driven: Network participants agree on the state of the ledger

The blockchain works by having network participants (nodes) validate transactions, group them into blocks, and add these blocks to the chain through a consensus mechanism.

Bitcoin is a decentralized digital cryptocurrency that operates on a peer-to-peer network without requiring intermediaries like banks or governments. Created by Satoshi Nakamoto in 2009, it was the first successful implementation of blockchain technology.

Key differences from traditional currency:

• Decentralized: No central bank or government control
• Limited supply: Maximum of 21 million bitcoins will ever exist
• Digital-only: No physical form
• Pseudonymous: Transactions are linked to addresses, not real identities
• Irreversible: Transactions cannot be reversed once confirmed
• Global: Works across borders without currency conversion

A hash function is a mathematical algorithm that takes input data of any size and produces a fixed-size string of characters (hash). In Bitcoin, the SHA-256 hash function is used.

Properties of cryptographic hash functions:

• Deterministic: Same input always produces same output
• Fast computation: Quick to calculate
• Avalanche effect: Small input change drastically changes output
• One-way: Computationally infeasible to reverse
• Collision-resistant: Nearly impossible to find two inputs with same output

Example:

Input: "Hello World"
SHA-256 Output: a591a6d40bf420404a011733cfb7b190d62c65bf0bcda32b57b277d9ad9f146e

Input: "Hello World!"
SHA-256 Output: 7f83b1657ff1fc53b92dc18148a1d65dfc2d4b1fa3d677284addd200126d9069

Importance in blockchain:

• Links blocks together
• Provides data integrity
• Enables proof-of-work mining
• Creates unique block identifiers