CPU (Central Processing Unit)
A central processing unit (CPU) is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logical, control and input/output (I/O) operations specified by the instructions.(Wikipedia)
Choosing the right CPU for your PC is extremely important because it is what helps your PC run. The main two types of processors are Intel and AMD. Choosing the right CPU for your build depends on what you intend to use your PC for. Intel has been the more popular out of the two, and is typically the go to choice when it comes to higher gaming, but in order to get a good one you need to be willing to spend over $300. An Intel i7 or higher is ideal. Where if you plan to use your PC for schoolwork and lighter games, a more budget AMD will work just fine. You can get an ideal one for this use for under $200.
In all, if you get a higher CPU, you should plan to spend a bit more on your other parts as well. Where if you get a lower end CPU, it doesn't make sense spending more on other parts.