Caching takes dynamic pages and turns them static. Technically speaking, it takes PHP pages that need to talk to the server and pre-renders them as HTML. This speeds up the delivery of the page to our browsers.
Plugin: W3 Total Cache
WordPress Plugin Page: http://wordpress.org/extend/plugins/w3-total-cache/
Install: Through WP Dashboard or FTP
Difficulty: Easy (with advanced settings)
Rating: High (2,000+ Ratings)
W3 Total Cache saves precious load time by caching pages which delivers them quicker. This plugin also has the capability to minify JS and CSS. It can also be used to store files on CDN.
*Note: it is highly recommended to go through each section of this plugins page and see which settings may or may not work for your site. Preview your site every time you make a change to see if anything is broken because of the settings.