Proof of work
In short Proof of work is a consensus mechanism where miners spend computing power solving puzzles to add blocks, making it costly to rewrite the chain's history.
Proof of work (PoW) is the method Bitcoin uses to agree on the order of transactions without a central authority. Miners compete to produce the next block, and the winner earns the block reward plus transaction fees. Other PoW chains include Litecoin, Dogecoin, Monero, and Ethereum Classic.
To produce a block, a miner bundles pending transactions and repeatedly hashes the block header with a changing number called a nonce, looking for a hash below a target value. There is no shortcut: finding one takes enormous numbers of attempts, but anyone can verify the result instantly. The network adjusts the difficulty so blocks arrive at a steady pace, about every ten minutes on Bitcoin, and nodes follow the valid chain with the most accumulated work.
This matters because rewriting history would require redoing that work faster than the rest of the network, which on a large chain means controlling most of the global mining power. That is what makes confirmed transactions on a big PoW chain very hard to reverse. Small PoW chains are a different story: with little hash power securing them, attackers can rent enough to run a 51% attack and double-spend coins.
The tradeoffs are energy use and specialized hardware, which tends to concentrate mining in large operations and pools. When judging a PoW coin, look at total hash rate, how concentrated it is across pools, and whether its algorithm shares hardware with a much bigger chain, since that makes renting attack power easier. Also note the emission schedule, including halvings that cut miner rewards over time.