The processor, central processing unit, or CPU is the core of any computer and handles most of the load of the necessary calculations that lie under the hood of PC gaming. Along with the graphics card, or GPU, the CPU has a major effect on the performance of the game as you experience it, and is perhaps the most important element of any gaming PC.
The best way to characterize CPUs is according to their power, which roughly corresponds with their price. CPUs tend to fall into three ranges: entry level, value level, and the cutting edge.
The newest and most powerful processors have the best stats on paper. However, they have the disadvantage of not offering good value. Generally, you will pay a large premium for the privilege of having the latest and greatest. At least for the current generation of CPUs, that much power is really not necessary to get really good performance on contemporary games. You are usually better off getting a cheaper processor, because you are likely to need to buy a new one in 2-3 years anyway. However, if you use your computer for serious work, graphical processing, or other resource-heavy roles, you might need the extra boost.
The middle range of gaming processors is where most of the best deals are. There is a wide variety of different CPUs here, because this middle tier of processors is a broad category. Normally, though, there are a few chips from each manufacturer that stand above the rest in terms of value. This can change rapidly, though, because CPUs tend to experience many small differences in prices over the course of the year. That means the best CPU to buy in this range could totally change over the course of a few months, especially when the main manufacturers release new CPUs and drop prices on the old ones. The CPUs in this range are the best buy for most buyers, because you will be able to get everything from a cheap processor able to run most things well to a somewhat more expensive chip that can run almost anything at high settings, although not necessarily maxed out settings.
There is a robust set of offerings of entry-level CPUs for people who are just getting started with building their own gaming PC. Entry-level CPUs are notably cheaper and offer less power, but they have another advantage: less dependence on support equipment. More powerful CPUs need cooling systems to prevent them from overheating, special power sources to give them enough energy, and advanced motherboards for installation. Not only do all of those elements cost more money, but they also make it more difficult to install and use the CPU. Entry-level products generally do not have this problem because they are less powerful. As a result, it is possible to buy these CPUs alone. You should still check to make sure that your choice is compatible with your motherboard and your operating system version, but aside from that, it should be easy to get going.
Things like the series designation, the overclock potential, the number of cores, and so on can all have a big impact on performance, and you won't see that if you zero in on the clock speed.
As the last point indicated, overclocking, or pushing a CPU beyond its manufacturer's limits, is a key element in getting the most out of your hardware. Not all CPUs are capable of this, and the performance gains come at the risk of damage to the chip, but it is often worth it.
Most CPUs have some restrictions on the motherboard that will fit them or the OS version and driver software you need for them to work correctly. Additionally, if the CPU needs accessories like cooling or a PSU, then you need to make sure those are compatible as well.
Especially for new CPUs, you need to do some research and make sure the unit will last. A CPU failure won't just make it impossible to get good game performance- your computer will not run at all.
The price of the same model of CPU can vary over a time as short as a month based on new product releases, higher or lower demand, and other factors. The difference generally won't be enough to let you move up a tier of quality for the same money, but you can get a nice deal on the CPU you already wanted if you pay attention.
There are two main manufacturers in the CPU market, just like the GPU market. One of them is the same: AMD. Instead of Nvidia, however, they compete against Intel. While AMD has a variety of offerings, Intel is famous for their popular "Core" series. For each generation of CPUs, the Core i3 is the low range of power, the Core i5 is the midrange, and Core i7 is the top end. In general, the i3 is entry-level and the i7 is the cutting edge, but there is a lot of variation within the i5 class. The "K" designation for the Core CPUs indicates the potential for overclocking, so the most recent generation's Core i5 K line is always a great place to start.
It is also important to note that AMD and Intel supply CPUs for laptops as well. These often have similar designations but much less power. Do your research and make sure you are buying the right product, because it is easy to get lost in all of the similar-sounding names and designations.