Portland cement is used to make concrete and can also be used to lay stone. It doesn't have any gravel in it. If you want to make cement with Portland cement, you have to add sand and gravel or you can even order what is called 3/4 mix. This would be if you're pouring a small slab, sidewalk, etc.
You can also use Portland cement if you are making mortar to lay stone. Portland would also work with garden art stuff.
Rich mortar is used when laying brick. It doesn't have any sand or gravel in it either.
The quick-crete stuff you buy at Lowes or Home Depot is for the homeowner that has a small cement job and you just add water. It does have the sand and gravel in it.
Did that make sense or did I add to the confusion???