What Is a CPU?

The central processing unit (CPU) is the computer component that's responsible for interpreting and executing most of the commands from the computer's other hardware and software.

Types of Devices That Use CPUs
All sorts of devices use a CPU, including desktop, laptop, and tablet computers, smartphones, even your flat-screen television set.
Intel and AMD are the two most popular CPU manufacturers for desktops, laptops, and servers, while Apple, NVIDIA, and Qualcomm are big smartphone and tablet CPU makers.
You may see many different names used to describe the CPU, including processor, computer processor, microprocessor, central processor, and "the brains of the computer."
Computer monitors or hard drives are sometimes very incorrectly referred to as the CPU, but those pieces of hardware serve entirely different purposes and are in no way the same thing as the CPU.

What a CPU Looks Like and Where It's Located?
A modern CPU is usually small and square, with many short, rounded, metallic connectors on its underside. Some older CPUs have pins instead of metallic connectors.
The CPU attaches directly to a CPU "socket" (or sometimes a "slot") on the motherboard. The CPU is inserted into the socket pin-side-down, and a small lever helps to secure the processor.
After running even a short while, modern CPUs can get very hot. To help dissipate this heat, it's almost always necessary to attach a heat sink and a fan directly on top of the CPU. Typically, these come bundled with a CPU purchase.

CPU Clock Speed
The clock speed of a processor is the number of instructions it can process in any given second, measured in gigahertz (GHz).
For example, a CPU has a clock speed of 1 Hz if it can process one piece of instruction every second. Extrapolating this to a more real-world example: a CPU with a clock speed of 3.0 GHz can process 3 billion instructions each second.

CPU Cores
Some devices use a single-core processor while others may have a dual-core (or quad-core, etc.) processor. Running two processor units working side-by-side means that the CPU can simultaneously manage twice the instructions every second, drastically improving performance.
Some CPUs can virtualize two cores for every one physical core that's available, a technique known as Hyper-Threading. Virtualizing means that a CPU with only four cores can function as if it has eight, with the additional virtual CPU cores referred to as separate threads. Physical cores, though, do perform better than virtual ones.
CPU permitting, some applications can use what's called multithreading. If a thread is understood as a single piece of a computer process, then using multiple threads in a single CPU core means more instructions can be understood and processed at once. Some software can take advantage of this feature on more than one CPU core, which means that even more instructions can be processed simultaneously.

Example: Intel Core i3 vs. i5 vs. i7
For a more specific example of how some CPUs are faster than others, let's look at how Intel has developed its processors.
Just as you'd probably suspect from their naming, Intel Core i7 chips perform better than i5 chips, which perform better than i3 chips. Why one performs better or worse than others is a bit more complex but still pretty easy to understand.
Intel Core i3 processors are dual-core processors, while i5 and i7 chips are quad-core. Turbo Boost is a feature in i5 and i7 chips that enables the processor to increase its clock speed past its base speed, like from 3.0 GHz to 3.5 GHz, whenever it needs to. Intel Core i3 chips don't have this capability. Processor models ending in "K" can be overclocked, which means this additional clock speed can be forced and utilized all the time; learn more about why you'd overclock your computer. Hyper-Threading enables the two threads to be processed per each CPU core. This means i3 processors with Hyper-Threading support just four simultaneous threads (since they're dual-core processors). Intel Core i5 processors don't support Hyper-Threading, which means they, too, can work with four threads at the same time. i7 processors, however, do support this technology, and therefore (being quad-core) can process 8 threads at the same time.

Due to the power constraints inherent in devices that don't have a continuous supply of power (battery-powered products like smartphones, tablets, etc.), their processors—regardless if they're i3, i5, or i7—differ from desktop CPUs in that they have to find a balance between performance and power consumption.