CPU, that's often called just "processor", is an abbreviation for Central Processing Unit. That's the core of every PC or hosting server, due to the fact that it performs each of the calculations and logical input/output procedures. Although the performance of a website or an application relies on other things as well, including the amount of physical memory or the connectivity of the server, the rate at which a particular processor works determines how quick an application will be executed. Later-generation processors have multiple cores which can severely improve their overall power and efficiency, as every core can take care of several processes independently or a few cores can take care of a single process that requires a sizeable computing power. Because each core functions at a certain speed, this architecture can be looked at as several individual processors cooperating.