I take it that you looked up the Data Types in the VBA help and that you would like to know the practical difference. In my experience, numbers of Double type sometimes don't evaluate to zero when one would expect, like
IF(SUM(3, 4, 13, -20) = 0, 0, 1)
This example doesn't show an error, I presume, but Double is capable of returning a non-zero result at the end of a zero-sum calculation. I haven't had a similar experience with Decimal and prefer to use it on principle ever since, which is a long time so that some of the details of the reasons have been forgotten.