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.