Kevin van Zonneveld has a quick new tutorial he's written up covering the installation and use of the Cache_Lite software to increase performance on your site.
Every time a request hits your server, PHP has to do a lot of processing, all of your code has to be compiled & executed for every single visit. Even though the outcome of all this processing is often identical for both visitor 21600 and 21601. So why not save the flat HTML generated for visitor 21600, and serve that to 21601 as well? This will relieve resources of your web server and database server because less PHP often means less queries.
The Cache_Lite PEAR package makes it simple to cache both entire pages or just small parts. He shows the installation (through aptitude on Ubuntu) and how to use it in a simple code example that stores the cached copy to the local drive for a given amount of time.