What is a CPU
CPU stands for Central Processing Unit. A CPU is often referred to as processor.
Examples of CPU’s are Intel i7 3770K, AMD Phenom II X4 965 Black Edition, PowerPC G5, and VIA C7-M 795.
A CPU usually sits on a PCB, which connects the pins or pads on the one side to the “die” on the other side. The “die” is the actual CPU, this is where the magic actually happens. The die can consists of one or more “cores”(which are the actual processors), several caches, a RAM controller, a PCI-e interface, and a GPU. The on-die configuration is different for every brand, model, codename, whatsoever, but think as the CPU as the Central Processing Unit, where everything you do passes through.
On the top left of the picture below you see an Intel CPU with a bare core(die) in the middle. On the top-right you see an AMD CPU with a heat-spreader, which spreads the heat from the core underneath it to a bigger surface to mount the heatsink on, and at the same time it protects the core. Also, below, you see the other side of the AMD CPU, with 939 pins which make the connection between the core and the motherboard.
Why do I need a fast CPU?
A fast CPU can significantly speed up your system, because it does (almost) all the calculations, unless it is “bottlenecked” by another, slower component.
A CPU is usually the component that the rest of the system is built around, because the choice between Intel or AMD, which socket and what number of cores makes a lot of difference in you choice of motherboard, PSU, RAM, etc.
The reason for a certain type of CPU is to find in the goal of the build. If you are building for an audio-editor or programmer, you’ll need a massive CPU to compile and render audio and code, but a less heavy GPU-setup than for a hard-core gamer. For lightweight applications such as text processing, browsing the web, and email, a dual-core CPU with a high clock-speed is sufficient, and there the real speed benefit is in the HDD.
So, a fast CPU is good, but keep in mind what you want the system to do, and that if your other parts, like motherboard, GPU, RAM and HDD are not of an equal quality, you’re not gonna use the full power of your CPU. Think of this when configuring or buying a new computer.