@Jeffro - They're actually fairly easy to install and come with great instructions. :)

When someone hits any page on your blog right now, PHP generates the page and gets data from the database. Both of those actions use CPU power.

Both caching plugins short-circuit that and instead serve a previously generated version of that page (which uses significantly less CPU). Two guests to your site will see the same content, so the plugin just generates the page for one and serves it to the second without using the CPU power needed to generate the page.

New comments and such automatically invalidate the cache files, so it's basically install and forget.