How does a consensus mechanism work?

Could you elaborate on how a consensus mechanism functions in the realm of cryptocurrency and finance? I'm particularly interested in understanding how various participants in a network come to an agreement on the state of the ledger or blockchain, without a centralized authority. What are the key steps involved in the process? Are there different types of consensus mechanisms, and if so, how do they differ from each other? I'd also appreciate any insights you have on the security implications and efficiency considerations of various consensus protocols.