Actually, I think you're making it too complicated. You know what a car is, right? It may surprise you, but you can't actually buy a "car". You can buy a Mustang, a Prius, or many other types of car. But there is no "car". The "car" is a concept, and Ford, Honda et. al. are all manufacturers who would love to sell you one. And they sell different types.
In computers, you know what an operating system is, but you're not running "operating system", you're running Windows, Linux or some specific type of operating system. You're viewing this message not with "HTML viewer", but FireFox, IE, Opera or some such.
Since you see this every day in every aspect of life, I find it odd that you're having difficulty with this one instance.