Intel's new fifth-generation chip was expected to be called the 586, following their earlier naming conventions. However, with the rise of AMD and Cyrix, Intel wanted to be able to register as a trademark the name of their new CPU, and numbers can't be trademarked. Thus, the Pentium was born. It is now one of the most recognized trademarks in the computer world, one reason why Intel doesn't seem to ever want to make another processor whose name doesn't have "Pentium" in it somewhere.
The Pentium is the defining processor of the fifth-generation. It has, in fact, had several generations itself. The first Pentiums were different, in many ways, compared to the latest ones. It had been the target for compatibility for AMD's K5 and Cyrix's 6x86 chips, as well as generations that have followed. The chip itself is instruction set compatible with earlier x86 CPUs, although it does include a few new (albeit rarely used) instructions.
The Pentium provides greatly increased performance over the 486 chips that precede it, due to several architectural changes. Roughly speaking, a Pentium chip is double the speed of a 486 chip of the same clock speed. In addition, the Pentium goes to much higher clock speeds than the 486 ever did. The following are the key architectural enhancements made in the Pentium over the 486-class chips (note that some of these are present in Cyrix's 5x86 processor, but that chip was developed after the Pentium).
The Pentium was the first superscalar processor; it used two parallel execution units. Some people likened the Pentium to being like a pair of 486s in the same chip for this reason, though this really isn't totally accurate. It is really only partially superscalar, because the second execution unit doesn't have all the capabilities of the first; some instructions won't run in the second pipeline. In order to take advantage of the dual pipelines, code must be optimized to arrange the instructions in a way that will let both pipelines run at the same time. This is why you sometimes see reference to "Pentium optimization". Regardless, the performance is much higher than the single pipeline of the 486.
Wider Data Bus
The Pentium's data bus is doubled to 64-Bit, providing double the bandwidth for transfers to and from memory, compared to the 486.
Much Faster Memory Bus
Most Pentiums run on 60 or 66MHz system buses; most 486s run on 33MHz system buses. This greatly improves performance. Pentium motherboards also incorporate other performance-enhancing features, such as Pipelined Burst Cache. The Pentium processor was also the first specifically designed to work with the (then new) PCI bus.
The Pentium uses Branch Prediction, to prevent pipeline stalls when branches are encountered.
Integrated Power Management
All Pentiums have built in SMM power management (which was optional on most of the 486s).
Split Level 1 Cache
The Pentium uses a split level 1 cache, 8 KB each for data and instructions. The cache was split so that the data and instruction caches could be individually tuned for their specific use.
Improved Floating Point Unit
The floating point unit of the Pentium is significantly faster than that of the 486.
The Pentium was available in a wide variety of speeds, as well as in regular and OverDrive versions. It was also available in several packaging styles, although the Pin Grid Array (PGA) is still the most prevalent. The original Pentiums, the 60 and 66MHz versions, were very different than the later versions that are used in most PCs; they used older, 5-Volt technology and encountered significant problems with heat. Intel solved this with later (75-200MHz) versions by going to a smaller circuit size and 3.3 volt power.
The Pentiums used three different sockets. The original Pentium 60 and 66 used Socket 4. Pentiums from 75MHz to 133MHz will fit in either Socket 5 or Socket 7; Pentium in 150MHz, 166sMHz and 200MHz flavours required Socket 7. Intel made Pentium OverDrives that allowed the use of faster Pentiums in older Pentium sockets (in addition to OverDrives that go in 486 Motherboards).
The Pentium processor achieved a certain level of "fame" as a result of the bug that was discovered in its floating point unit not long after it was released. This is commonly known as the "FDIV" bug after the instruction (Floating Point Divide) that it most commonly turns up in. While bugs in processors are relatively common, they usually are minor and don't have a direct impact on computation results. This one did, and achieved great notoriety in part because Intel didn't own up to the problem and offer to correct it immediately. Intel did eventually offer a replacement on affected processors, which were only found in early versions (60 to 100MHz) sold in 1994 and earlier.
If you suspect your Pentium of having the FDIV bug, try this computation test using a spreadsheet or calculator program: take the number 4,195,835 and divide it by 3,145,727. Then take the result and multiply it by the same number again (3,145,727). You should of course get the same 4,195,835 back that you started with. On a PC with the FDIV bug you will get 4,195,579 (an error of 256), but beware that some operating systems and applications have been patched to compensate for this bug, so a simple math test isn't necessarily conclusive.
For many years, the Pentium processor was the mainstream processor of choice, but finally the Pentium MMX drove it to the economy market. With the regular Pentium maxing out at 200MHz and the Pentium MMX 166MHz dropping well below $200, the "Pentium Classic" didn't make nearly as much sense as it used to for new PCs. The 60 and 66 soon became obsolete, due to their slow speed and older technology, and the 75MHz to 150MHz versions went obsolete soon after, because their performance was much lower than the 166MHz and 200Mhz versions, that shipped for almost the same amount of money.
The entire Classic Pentium line is now obsolete, firstly due to the availability of inexpensive, faster Pentium MMX chips (as well as comparable offerings at the time from AMD and Cyrix). The non-MMX Pentium is no longer used in new systems. However, since the Pentium MMX required split rail voltage, the classic Pentium 200MHz remained a great chip for those who had Socket 7 [[Motherboard]s and who wanted to upgrade, but who did not have split rail voltage support.
Current-grade processors are capable of exponentially better processing. It is cheaper today to buy a 10x more powerful multi-core processor than it was to buy the first Pentiums.