• Core - The Celeron chip is based on a
  Pentium 4 core.
  
• 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 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 depending on what you want to do with your computer. If all you do is check e-mail and browse the Web, the Celeron is fine, and the price difference can save you a lot of money. 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.