Server-side caching is a lot more interesting than client-side, at least for me. Server-side caching saves content such as images, HTML, and CSS on the server that's hosting the website, as opposed to storing it in visitors browsers. With the server-side caching, the software doing the caching will check if there's a cached version of the current page before sending it to the browser for rendering. There's a plethora of WordPress plugins that offer various caching features.
One of the most popular caching plugins for WordPress is W3 Total Cache . I attribute its popularity to its rich feature set. It does a lot, from minifying your JavaScript and CSS code, to integrating seamlessly with paid CDNs. W3 Total Cache is a complex plugin and a little difficult to figure out at first.
I've never used W3 Total Cache, it's always seemed to be a bit of an overkill for most of my websites. If you are interested in more details about W3 Total...