Consensus mechanisms are usually described as securing a network, which understates what they do. They settle the order of events among participants who cannot trust each other.
Ordering is the underlying problem
If two conflicting transactions spend the same funds, the network must agree which came first. Without a single authority, there is no obvious way to establish that.
Participants are spread across the world with different network delays, so timestamps alone cannot settle it. Someone must be selected to propose the next agreed batch.
Consensus rules define how that proposer is chosen and how everyone else confirms the result. Everything else follows from those two questions.
Selection must also be costly in some form. If proposing were free, anyone could flood the network with competing versions of history and no version would be authoritative.
Proof of work makes proposal costly
In a work-based system, proposers compete by expending computation to find a solution to a difficulty target. Finding it grants the right to propose the next block.
The cost is what provides security, because rewriting history would require redoing that expenditure faster than the rest of the network extends the chain.
Energy consumption is not incidental to this design. It is the mechanism through which the cost of dishonesty is imposed.
Proof of stake substitutes capital for energy
Stake-based systems select proposers in proportion to assets committed to the network. Misbehaviour can result in that stake being reduced or removed.
The security argument is similar in structure but different in substance. The cost of attacking is the capital at risk rather than the resources consumed.
This lowers energy use substantially while introducing different considerations around concentration of stake and the mechanics of penalties.
Finality differs between designs
Some systems treat confirmation as probabilistic, with the chance of reversal falling as blocks accumulate. A transaction becomes settled by convention rather than by rule.
Others provide explicit finality, where a block confirmed under defined conditions cannot be reverted without violating the protocol.
The distinction matters for anyone deciding how long to wait before treating a payment as complete.
Why the choice shapes everything else
Consensus determines throughput, cost, energy use and the assumptions a user must accept about who could disrupt the network.
Comparisons between chains often focus on speed, which is downstream of these choices rather than an independent property.
Understanding what a mechanism is deciding, and at whose cost, is more informative than any single performance figure.