The blockchain network being decentralized has to have a mechanism to agree upon the next state of system. This is achieved via a consensus protocol.
Types of Consensus Mechanism
Over the last two decades, several new consensus protocols have been developed and used in the blockchain space. The most common ones are:
BFT: Byzantine Fault Tolerance Protocol
This is the most classical way to reach consensus which works on the assumption that the network has at most ⅓ of malicious nodes. The protocol requires each honest node to agree on the state via voting. This involves several rounds of communication among the nodes.
Nakamoto consensus was pioneered by Bitcoin. It uses proof-of-work and longest-chain-win rule to reach consensus among all honest nodes. Using proof-of-work, each node is required to solve a computational puzzle and the node fastest to find a solution decides the next state of the system. In the longest-chain-win rule, the node will adopt the longest chain and extend it with new valid block. A block in the blockchain is deemed to be committed once there is a sufficient number of block mined after the block.
PoS: Proof of Stake
In Proof-of-Stake, nodes are required to stake an amount of assets (e.g., native currency of the blockchain). Those who stake are allowed to decide on the next state of the system. By putting a stake, the expectation is that they will not make detrimental decisions. And in fact, the probability that a certain node is chosen to propose the next state is proportional to its stake.