Here is a brief history of the important CPUs which were featured in PCs over the years. Although it has been dominated by Intel, we can see AMD coming out with some cutting-edge CPUs in the last few years.

Processor	Description	Core Photo
Intel 4004 (1971)	Intel's first microprocessor. It was a breakthrough in computer technology, powering one of the first electronic calculators.
Intel 8008 (1979)	This is the first chip that was used in a PC. It could run at 4 MHz and supported up to 1 MB of system RAM.
Intel 80186 (1980)	The 186 was a very popular CPU. There are two versions, an 8-bit or 16-bit (the amount of bits allowed to be inputted each clock cycle). The 186 eventually reached a speed of 25 MHz, using 3 volts.
Intel 80286 (1982)	This is a 16-bit processor which supports up to 16 MB of RAM. It was the first processor to be able to multitask (run multiple programs at a time), but the operating systems at that time could not take advantage of it. The chip ran as high as 20 MHz.
Intel 80386 (1988)	This was a revolutionary chip for the PC industry. It was the first 32-bit processor, which meant it could use twice as much data on each clock cycle. The 386 was also capable of using 16 bytes of cache. Having speeds from 12.5 MHz to 33 MHz, it was a big step for the PC, and was very user-friendly.
Intel 486 (1991)	The 486 used much of the 386 architecture, but it added a math coprocessor, which made it much faster. It can go up to 120MHz. It also came in an SX version, which was cheaper to make because no math coprocessor was utilized.
Intel Pentium (1993)	The Pentium was available in speeds from 75MHz all the way up to 233MHz. It had an FPU, which allowed much grater performance. Internally, it had two 32-bit chips which split the work. The chip came with 16 KB of cache.
Intel Pentium II (1997)	Built with over 7.5 million transistors, this processor included MMX technology, used to process video and audio faster. This CPU used a cartridge to connect to the motherboard. It had a built in L2 Cache, making it a very fast processor.
AMD Athlon (1999)	The first processor that was faster than its Intel counterpart, the Athlon made history. It featured 256Kb of cache, as well as 3DNow instructions designed to improve the FPU. It uses a 266MHz bus, twice as fast as the Pentium III. AMD made history with the Athlon by being the first to break the 1GHz barrier.	© 2001 by AMD.
Intel Pentium III (1999)	Performing similarly to the AMD Athlon, the Pentium III features 256Kb of cache. It comes in speeds from 450MHz all the way up to 1.13 GHz.
Intel Pentium 4 (2001)	This chip is actually much slower than its predecessor, but the MHz advantage it has over the Pentium III made it faster.  It is especially fast on internet applications, although it cannot compete with its AMD counterpart in any other area.
AMD Thunderbird (2001)	Currently the best PC processor on the market, AMD dominates the world of 3D gaming as well as professional programs. It comes in first in almost every test, even though its clock is about 0.5GHz slower than the Pentium 4.	© 2001 by AMD.