Here's my definition, for what its worth.  Cloud Computing is the separation of the computing resources from the client by the Internet.

This implies that, just like in a real corporate server environment, all the primary computing services are located in a server bunker, properly resourced and protected.  It also means that, just like in a real corporate environment, users connect to these services by way of a data service.  Where it differs however, is that in Cloud Computing, the data service is a free and open Internet whereas for the corporate, it is generally some point-to-point dedicated channel from the user to the server (note that I'm choosing to not distinguish between corporate in-house and co-lo data centres – really it doesn't much matter).

So, what does Cloud Computing mean for users? 

The first thing it means is that they have no visibility of the physical structure / configuration of the data centre – long gone are the days of wandering over to the rack and re-patching your own connection.  But, what visibility do they actually need? 

Does it matter if there are separate servers for file & print and for email or whether they are the same physical box? 

Does it even matter if the physical servers actually support applications for multiple companies?