What is the difference between a Pentium and a Celeron processor?

Here are the most important similarities and differences between the Pentium 4 and the Celeron chips coming out today: 

• Cache- Celeron chips have less cache memory than Pentium 4 chips do. A Celeron might have 128 kilobytes of L2 cache, while a Pentium 4 can have four times that. The amount of L2 cache memory can have a big effect on performance.
• Clock speed - Intel manufactures the Pentium 4 chips to run at a higher clock speed than Celeron chips. The fastest Pentium 4 might be 60 percent faster than the fastest Celeron.
• Bus speed - There are differences in the maximum bus speeds that the processors allow. Pentium 4s tend to be about 30 percent faster than Celerons.

    When you sort all this out and compare the two chips side by side, it turns out that a Celeron and a Pentium 4 chip running at the same speed are different beasts. The smaller L2 cache size and slower bus speeds can mean serious performance differences. If all you do is check e-mail and browse the Web, the Celeron is fine,If you want the fastest machine you can buy, then you need to go with the Pentium 4 to get the highest clock speeds and the fastest system bus.
 
Another Review

    The differences between Pentium M and Celeron M are basically the external clock rate and the amount of memory cache:

• Celeron M: 400 MHz external clock, L2 cache of 512 KB up to model 340 (and also on 353 and 373 models) and L2 cache of 1 MB on model 350 up.
• Pentium M: 400 MHz external clock on the models ending with a “3” or “5” and 533 MHz on models ending with a “0”. L2 cache of 2 MB from model 715 up (except model 718, which has 1 MB) and 1 MB on all other models.
  

    At first calling Celeron M as Centrino wouldn’t be a problem. However, this causes certain confusion on the market. A “1.6 GHz Centrino” is faster than a 1.6 GHz Celeron M, since a Pentium M is faster than a Celeron M running at the same clock rate. The problem is the user buying a 1.6 GHz “Celeron M Centrino” thinking that it will achieve the same performance of a “real” 1.6 GHz Centrino, which won’t happen