What is the difference between a Intel celeron processor 900 and a Intel Pentium duo core processor?

In general, Core 2 Duo are high performance processors, with two "CPUs on a chip" (that's the "core duo" meaning.

While Celeron processors are, generally, single core, low to medium performance, intended for budget to moderate mobile (laptop) use. Lower power, single CPU, less performance (but uses less battery power in that).

Note, having 2 cores is usually a plus, as the CPU has two 'brains' and can do two things at once. But some software is not two-core-aware, so in those cases the two cores wont really be used, and the chip will act like an older style single core.

Bottom line:

So if you need high performance, go for the Core 2 Duo (it will be at a higher price than Celeron, and will use more electrical current).

If performance is not a concern, but you need lower cost, or longer battery life, the Celeron may be a good choice.