CPU, that's frequently called just "processor", is an abbreviation for Central Processing Unit. That's the core of each and every laptop or computer or web server, since it executes all of the calculations and logical input/output functions. Even though the performance of a site or an application depends on other things as well, like the amount of physical memory or the connectivity of the web server, the rate at which a particular processor works determines how quickly a program shall be executed. Later-generation processors have several cores which can drastically increase their overall power and efficiency, because each core can manage numerous processes separately or several cores can deal with a single process which needs a sizeable processing power. Because every single core operates at a particular speed, this architecture can be looked at as a number of separate processors cooperating.