What is the difference between whole grain mustard and regular yellow mustard?
Yes, there is (generally) a significant taste difference between whole grain mustard and regular yellow mustard.  Texture plays a part of it, but it is secondary to the overall difference in flavor.

I say generally because there's a lot of variation in each, and some extremes of one type may approach some extremes of the other type in flavor.

Yellow mustard is usually very mild, and vinegar plays a very strong role in the flavor.

Whole grain mustard is usually stronger than yellow, and the vinegar flavor is much less pronounced, if not altogether absent.

In many situations, one type can usually be substituted for the other, although it is a matter of personal preference, and many people will argue that some uses of mustard are appropriate for one type, but not the other (for example, I would never use yellow mustard as a condiment for cheese and crackers, but I would use a whole grain mustard for some types of cheese).