APC, or Alternative PHP Cache, is a PHP module which caches the output code of database-driven script apps. Dynamic PHP sites keep their content inside a database which is accessed whenever a visitor opens a page. The content that needs to be shown is gathered and the code is parsed and compiled before it's delivered to the site visitor. All of these actions need some processing time and require reading and writing on the hosting server for every single page which is accessed. While this cannot be avoided for Internet sites with regularly changing content material, there're a lot of Internet sites that feature the same content on a lot of of their webpages at all times - blogs, info portals, hotel and restaurant Internet sites, etc. APC is very useful for this kind of sites as it caches the already compiled code and displays it any time visitors browse the cached pages, so the code doesn't have to be parsed and compiled all over again. This will not only lower the server load, but it will also increase the speed of any website a few times.