Proof of Work is a consensus algorithm used by blockchain networks, including Bitcoin and Ethereum. Proof of Work operates through a hashing competition, where each node competes to find a hash value for a block that is less than the dynamically-adjusted target. The first node to generate a valid hash processes the block and propagates it to the network, typically receiving a token reward as compensation for their work. This iterative, randomized process for transaction validation prevents spam and denial of service attacks. There are a variety of hashing algorithms used by Proof of Work systems, including SHA-256 (Bitcoin), Equihash (Ethereum), and Scrypt (Litecoin).