APC (Alternative PHP Cache) is one of the orthogonal tools you can use to speed up the execution of PHP code. This article explains from scratch the correct use of APC's system cache (not touching the user cache, which is just a standard key/value map). APC has also other features like upload progress support, but the system cache is its main features and has such a little footprint on your code that you should throw an apc.php file to everyone picking on you for writing strings in single or double quotes.

He describes what the APC cache does for you (both on a lower, opcode and a higher, performance levels) and how it relates to the shared processes a web server uses. He finishes off the post with a quick guide to getting it installed and working ("pecl install") and some of the common settings to tweak to get the most of the functionality.