What is the Difference Between These Processors?

[XxGodJrxX]

Quick question. What is the difference between a Pentium 4 processor and a Pentium D processor. I have been shopping around for a computer and don't really know the difference between these two. Thanks in advance.

In this instance I believe the 'D' refers to the fact that the processor is a dual core processor (two processors mounted onto the same die), though this does not hold true for the family of Celeron D's.