APC, or Alternative PHP Cache, is a PHP module which caches the output code of database-driven script applications. Dynamic PHP websites store their content inside a database which is accessed whenever a visitor loads a page. The content that needs to be viewed is gathered and the code is parsed and compiled before it's delivered to the visitor. All of these actions take some processing time and require reading and writing on the web server for each page which is accessed. While this cannot be avoided for websites with constantly changing content, there're numerous Internet sites which have the exact same content on a lot of of their pages all of the time - blogs, informational portals, hotel and restaurant Internet sites, etc. APC is really useful for such sites because it caches the already compiled code and shows it any time visitors browse the cached webpages, so the code does not have to be parsed and compiled repeatedly. This will not only greatly reduce the server load, but it will also increase the speed of any site several times.