The CPU Can be compared with the human brain which does the reasoning task.
Alternately referred to as a processor, central processor, or microprocessor, the CPU (pronounced sea-pea-you) is the Central Processing Unit of the computer. A computer’s CPU handles all instructions it receives from hardware and software running on the computer.

Computer CPU (Central Processing Unit)
Tip: The CPU is often referred to as the brain of the computer. However, it is more appropriate to refer to software as the brain and the CPU as a very efficient calculator. A CPU is really good with numbers, but if it wasn’t for the software it wouldn’t know how to do anything else.
Note: Many new computer users may improperly call their computer and sometimes their monitor the CPU. When referring to your computer or monitor, it is proper to refer to them as either the “computer” or “monitor” and not a CPU. The CPU is a chip within the computer.
The picture below is an example of what the top and bottom of an Intel Pentium processor may look. The processor is placed and secured into a compatible CPU socket found on the motherboard. Processors produce heat, so they are covered with a heat sink to keep them cool and running smoothly.
Computer CPU (Central Processing Unit)
As you can see in the above picture, the CPU chip is usually in the shape of a square or rectangle and has one notched corner to help place the chip properly into the CPU socket. On the bottom of the chip are hundreds of connector pins that plug into each of the corresponding holes in the socket. Today, most CPU’s resemble the picture shown above. However, Intel and AMD have also experimented with slot processors that were much larger and slid into a slot on the motherboard. Also, over the years, there have been dozens of different types of sockets on motherboards. Each socket only supports specific types of processors and each has its own pin layout.
Intel 4004 processor

CPU history

The CPU was first developed at Intel with the help of Ted Hoff and others in the early 1970’s. The first processor released by Intel was the 4004 processor, shown in the picture to the right.
CPU history.
Components of the CPU

In the CPU, the primary components are the ALU (Arithmetic Logic Unit) that performs mathematical, logical, and decision operations and the CU (Control Unit) that directs all of the processors operations.
Machine cycle
Over the history of computer processors, the speed (clock speed) and capabilities of the processor have dramatically improved. For example, the first microprocessor was the Intel 4004 that was released November 15, 1971, and had 2,300 transistors and performed 60,000 operations per second. The Intel Pentium processor has 3,300,000 transistors and performs around 188,000,000 instructions per second.

Types of CPUs

In the past, computer processors used numbers to identify the processor and help identify faster processors. For example, the Intel 80486 (486) processor is faster than the 80386 (386) processor. After the introduction of the Intel Pentium processor (which would technically be the 80586), all computer processors started using names like Athlon, Duron, Pentium, and Celeron.
Today, in addition to the different names of computer processors, there are different architectures (32-bit and 64-bit), speeds, and capabilities. Below is a list of the more common types of CPUs for home or business computers.
Note: There are multiple versions for some of these CPU types.

AMD processors
Athlon XP
Athlon 64
Mobile Athlon 64
Athlon XP-M
Athlon 64 FX
Turion 64
Athlon 64 X2
Turion 64 X2
Phenom FX
Phenom X4
Phenom X3
Athlon 6-series
Athlon 4-series
Athlon X2
Phenom II
Athlon II
E2 series
A4 series
A6 series
A8 series
A10 series
Intel processors

80286 (286)
80386 (386)
80486 (486)
Pentium w/ MMX
Pentium Pro
Pentium II
Pentium III
Pentium M
Celeron M
Pentium 4
Mobile Pentium 4-M
Pentium D
Pentium Extreme Edition
Core Duo
Core 2 Duo
Core i3
Core i5
Core i7
The AMD Opteron series and Intel Itanium and Xeon series are CPUs used in servers and high-end workstation computers.
Some mobile devices, like smartphones and tablets, use ARM CPUs. These CPUs are smaller in size, require less power, and generate less heat.

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culled from http://www.computerhope.com/jargon/c/cpu.htm