While WP-Cache caches to static HTML files, it still needs the PHP engine to be started up to serve those static HTML files. WP Super Cache solves this, by rerouting the request past the PHP engine with mod_rewrite, and serves the HTML file directly.
WP Super Cache gets around that. When it is installed, html files are generated and they are served without ever invoking a single line of PHP. How fast can your site serve graphic files? That’s (almost) as fast it will be able to serve these cached files. If your site is struggling to cope with the daily number of visitors, or if your site appears on Digg.com, Slashdot or any other popular site then this plugin is for you. #
I must say, this is an ingenious extension to the original WP-Cache idea. This plugin is definitely a must for big sites, but it might be overkill for smaller sites (like mine).