OrderMaximumQuantity (Maximum cart qty) : the maximum quantity of a product a customer may purchase at one time.
E.G. assume your store sells beer and there is a legal limit of 24 beers per purchase. Set the OrderMaximumQuantity = 24 and a customer may purchase 1-24 beers, but not 25 or more beers.
AllowedQuantities : the specific quantities a customer may purchase.
E.G. beer comes in 6 packs, 12 packs, 18 packs, and cases of 24. Set AllowedQuantities = 6,12,18,24 and a customer may purchase those quantities, but not anything like 3, 7, or 13 beers that would force you to split up the packaging.
I find OrderMaximumQuantity extremely useful for my download products. I set OrderMaximumQuantity = 1 so the customer cannot accidentally purchase more than 1 of a digital file.