Central Processing Unit (CPU)
The CPU is the "brains" of the whole operation. In a nutshell, a CPU does a lot of mathematical computations, really, really quickly, all inside of a single second. Put another way, a CPU operates in "cycles" and the clock speed measures how many cycles per-second a CPU can perform. The more cycles per second, the faster the CPU.
Here's a picture of the top side of a CPU for a visual reference:
And the bottom of a CPU looks like this:
Unless you're building your own computer, server or some other system, you'll likely never see a CPU directly. The CPUs you'll interact with will be virtual and abstracted away through a technology known as Virtualisation.
Right now, the thing to know about CPUs is they're incredibly complicated. Therefore, you only really need to know about a few high level concepts to do your job as a systems administrator in a Cloud/virtualised environment.