With increasing amounts of data and server-side processing, caching systems are as important as ever. There are so many varieties of these systems out there, it can be confusing if you don’t understanding the different nuances. Worse yet, if you pick the wrong caching system for your needs or fail to optimize it correctly, the results could be disastrous. There are two broad types of cache when it comes to web sites and web applications. Client side, and server side.
Source: Web Cache: APC vs Memecached vs Varnish | Neon Rain Interactive
See https://blog.2020media.com/2015/06/2020media-can-handle-stephen-fry/ for a case-study on using Varnish to handle a peak traffic hit for a web hosting client of 2020Media.