What is Proof of Work (PoW)?

Proof of Work (PoW) is the original blockchain consensus mechanism invented for Bitcoin. Miners compete to solve a computationally intensive cryptographic puzzle: find a nonce such that the block header's SHA-256 hash is below a target value (has N leading zeros). The puzzle is asymmetric: hard to solve, trivial to verify. The first miner to find the solution broadcasts the block; other nodes verify and accept it; the winning miner receives the block reward (newly minted BTC) + transaction fees. Key properties: (1) Security through energy expenditure — attacking PoW requires controlling 51% of the network's hash rate, which is enormously expensive; (2) Difficulty adjustment — Bitcoin adjusts mining difficulty every 2016 blocks (~2 weeks) to maintain ~10 minute block times as hash rate changes; (3) Energy intensive — Bitcoin's PoW uses more electricity than many countries (estimated ~130 TWh/year); (4) ASIC dominance — specialized mining hardware (ASICs) centralize mining power; (5) Sybil resistant — identity attack is prevented by the cost of compute.