The APC PHP extension is a PECL extension that saves compiled versions of your code in memory, in order that your PHP code is not compiled on every page load.

Below I provide an overview of how I configured APC on my Debian server to improve the performance of my Drupal site.

My Debian server was already running Apache 2 and PHP 5, so installing PHP APC was as easy as doing the following:
apt-get install php-apc