If you are a system administrator and you want to push the performance of your PHP website such as Drupal, WordPress or wrote by you or other programmers, there are good chances that you’ve heard about the Alternate PHP Cache or simply APC.
“The Alternative PHP Cache (APC) is a free and open opcode cache for PHP. Its goal is to provide a free, open, and robust framework for caching and optimizing PHP intermediate code.”
That everything you’ll read about APC on the php.net site, and i think is not enough in comparison to the huge boost of performance that this small component can give you, for some quick facts check the tests done by Guillermo Garron on his website.