Proof of Work (PoW) is the original blockchain consensus mechanism, introduced by Bitcoin and still used by Bitcoin, Litecoin, Dogecoin, Monero, and a number of other networks. Miners compete to solve a computationally difficult puzzle — repeatedly guessing numbers until one produces a result meeting the network's difficulty target. The first miner to find a valid solution gets to add the next block of transactions to the chain and is rewarded with newly issued coins plus transaction fees.
Why it exists
The "work" in Proof of Work is what makes the system secure: producing a valid block requires a real, externally verifiable expenditure of computing power and electricity, which makes rewriting history or attacking the network prohibitively expensive — an attacker would need to out-compute the rest of the network combined. This is a genuinely different security model from Proof of Stake, which secures the network through capital at risk rather than energy spent.
Why it matters for Shariah screening
Mining itself is not generally treated as a Shariah concern — a miner is performing a genuine service (processing and securing transactions) and is compensated for real computational work and cost, which doesn't resemble riba, gharar, or maysir in its basic structure. See Is Crypto Mining Halal? for the fuller analysis, including the environmental and resource-use questions some scholars raise separately from the core contract structure. Where PoW becomes relevant to a coin's overall screening is less about the mechanism itself and more about what's built on top of it — the same way a Layer-1's own mining doesn't inherit riba concerns from third-party lending apps built on the chain.