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.
A modern CPU is usually small and square, with many short, rounded, metallic connectors on its underside. Some older CPUs have pins instead 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.
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 that has a clock speed of 3.0 GHz can process 3 billion instructions each and every second.
Some devices have a single-core processor while others may have a dual-core (or quad-core, etc.) processor. Having two processor units working side-by-side means that the CPU can manage twice the instructions every second, simultaneously, drastically improving performance.
Some CPUs can virtualize two cores for every one physical core that's available, known as Hyper-Threading. This means a CPU that may only have 4 cores can function as if it has 8, with the additional virtual CPU cores, referred to as separate threads. Please know, however, that physical cores do perform better than virtual ones.