The most common type of consensus mechanism is proof of work (PoW). PoW is used by many popular cryptocurrencies, such as Bitcoin. In a PoW system, miners compete to solve complex mathematical problems. The first miner to resolve the problem gets to add the next block of transactions to the blockchain. The advantage of PoW is that it is very secure. The disadvantage of PoW is that it is energy-intensive and can be slow.