How to understand some key system consistency algorithoms

When we design a system, we may want our systems to be consistency, scalability and so on. Currently, there are some famous consistency algorithms. How to understand them easily.

1, Paxos and its extensions
2, Replicated State Machine mechanisms
3, Quorum

Welcome to adding other famous consistency algorithms and its understanding ;-)

Reading text books or papers may be the only method to understand the consistency models and algorithms.

For list of important models/algorithms, this blog post Distributed systems theory for the distributed systems engineer and this page contain links to papers and webpages.