Pentium vs. Celeron

I don't know enough about PC hardware to figure this stuff out.
And I have a feeling it'd take me longer to search the net for this.
So what's the big difference between a P4 and a CeleronD.
I ask this silly question because I might be getting a new CPU shortly.
A P4 2.4Ghz 1MB/533FSB or a CeleronD 3.04Ghz 256K/533FSB.
(Both roughly the same price)
Laugh if you wish, but I can't learn if I don't ask.
And I have a feeling it'd take me longer to search the net for this.
So what's the big difference between a P4 and a CeleronD.
I ask this silly question because I might be getting a new CPU shortly.
A P4 2.4Ghz 1MB/533FSB or a CeleronD 3.04Ghz 256K/533FSB.
(Both roughly the same price)
Laugh if you wish, but I can't learn if I don't ask.