the difference is a celeron is a budget processor. it has been stripped down and will not perform like the p4. the cpu's instructions have been inhibited and like shumway said they have a much smaller L2 cache
ok heres a comparison. A 3.06 GHZ celeron D processor performs around the speed of a 2.4-2.6 GHZ pentium. Sometimes lower for applications where l2 cache is important.