APC, or Alternative PHP Cache, is a PHP module that caches the output code of database-driven script software apps. Dynamic PHP websites hold their content inside a database that is accessed whenever a visitor opens a webpage. The content that has to be shown is retrieved and the code is parsed and compiled prior to it being delivered to the visitor. All of these actions need some processing time and require reading and writing on the hosting server for every single page which is loaded. While this can't be avoided for websites with regularly changing content, there're a lot of websites that present the exact same content on a number of of their webpages all of the time - blogs, info portals, hotel and restaurant sites, and many others. APC is really useful for this type of websites as it caches the already compiled code and shows it every time visitors browse the cached pages, so the code doesn't need to be parsed and compiled again. Not only will this lower the server load, but it'll also increase the speed of any Internet site many times.