The reason for the Celerons existence owes to the advent of an AMD processor called the K6. AMD brought out a low priced alternative to the Pentium, and Intel, instead of drastically reducing their pricing on their "meat and potatoes" processor to meet that of the lower priced K6 (and eventually, the Athlon), introduced a "crippled" version of the Pentium, and called it the Celeron. (In response, AMD came back with an even lower priced processor of appalling performance called Duron, but that is another story)
Processors are commonly classified by their "Core" speed -- ergo, a 933 MHz Pentium III processor has a core speed of 933 MHz. There are however other significant features that are "classifiable" within a processor -- namely "bus" or "memory access" speed, and onboard "Cache" size (and speed). The Bus speed is the speed at which the processor fetches data from memory, and places it in the onboard Cache, and also represents the speed at which it communicates with its peripheral chipset. While each processor model operates at its own "native" Core speed, the bus speeds vary according to the motherboard (peripheral) Chipsets and memory with which they are designed to work. Likewise the cache size will vary from implementation to implementation. This distinction will become significant in as we move on.
The Celeron's basic design is identical to the Pentium. Actually, it is said that the die used to produce them is identical to thatof the equivalent Pentium. It is also said that they actually were Pentiums that failed certification (performance testing). Their marketing, and a small "adjustment" imposed during the final stage of production, is what makes the difference between the Pentium and Celeron. For the equivalent core speed Celeron, Intel "constrains" certain features. If the equivalent Pentium has a 512K cache, the Celeron will have only 128 or 256Kof the 512K cache enabled. If the Pentium has a 133 MHz bus speed , the Celeron is be limited to 66 MHz. This implies that not only is the cache memory reservoir from which the processor draws its data and instructions 1/4 to 1/2 the size of a Pentium, but the rate at which it is refilled is 1/2 thatof the Pentium. So metaphorically, you'd have a straw filling a pail, as opposed to a 3" fire hose filling a swimming pool.
One only need examine the trends in Intel processor design to see the significance of cache and bus speed. Recently, Intel took a revamped PIII core, goosed its bus speed to 400 MHz from 133, increased its cache size to 1 Megabyte, and achieved performance with near equivalence to a P4, while running it at significantly lower core speed -- it now markets it under the name "Centrino," a performance notebook processor that sips power compared to a standard PIII. Similarly, the first P4 processors had only 256K cache & a 400 MHz bus, but were quickly boosted to 512K cache with a 533 MHz and then 800 MHz bus . Now, the cache size is shortly to become 1 Megabyte, with 2 and 3 Megabyte cache chips on the horizon. On the power server side, the Pentium Xeon chips already have 2, 3, 4, and 6 Megabyte onboard caches.
Now when someone says "Unless you are doing specialize work you won't notice the difference" he is dead wrong, unless they are comparing a Celeron with a core speed of 2.2 GHz with a Pentium with a core speed of 1.6 GHz. Ergo, comparing Apples and Apples, a 2 GHz Celeron is never a match for a 2 GHz Pentium, and the difference would not only be measurable, but noticeable -- sometimes profoundly so
Also, Intel uses "staged" marketing. While a 3 GHz Pentium may be the leading edge of the Pentium Market, the fastest Celeron available may only be a 2.2 GHz. Therefore, a Pentium and Celeron with the same core speed are never introduced at the same time. So, you see, there can be no real comparison of equivalent Celerons and Pentiums except retrospectively. When the Celeron of one speed is introduced, the Pentiumof the same speed becomes "obscured" by virtue of the introduction of far faster Pentiums with improved technology as in the case of hyper-threading technology -- there are no Hyper-threading enabled Celerons
To make the point clear, in the auto industry, desirable features are introduced in the premium lines, and only filter down and become available on the economy lines after years pass. To ever say that at a point in time, the Lincoln Continental is comparable in feature and performance to a Ford Focus (same manufacturer), is fanciful at best. This is not to say that both autos do not perform according to their unique design, or are not suited to a particular use, -- I would never intentionally purchase a Celeron, knowing that not unlike cars of old, that my father had installed a "governor" that might limit functionality as well as my use, while outwardly appearing to be no different than any other car on the road -- unless I fully understood that at the point of purchase and price was the major consideration.
The one true measure that can be applied to the Celeron vs. Pentium issue is that of Price/Performance. If the Celeron and the Pentium were plow horsesof the same size, the Pentium could do a factor of 1x times the amount of work that the Celeron could accomplish in the same time frame, therefore favoring the valueof the Pentium But, if you measure the amount of work accomplished by each horse against the cost of each horse, the economics would favor the Celeron.