Back to the original question...
Clustering is when you put a lot (or a few if you want) of computers in LAN and they share the tasks, using all of the processors. I think they need to have an OS designed for this, but I'm not sure.
Scalability is about the total processor power you get from all of them together. Linear Scalability is, for example, when you put two 2Ghz computers together and the total speed is 4Ghz. However, this is more of an idea, than an achievable goal since they need to comunicate, share memory and other stuff that consumes cycles.
A good scalable computer is a computer that wastes few cycles communicating. A bad scalable computer is one that wastes a lot of cycles.
Hope this is clear enough for you.