Hmm, at least I would not have been able to explain that to somebody asking me for it. But Ars Technica has jumped into the niche and written a neat article about what .NET is, about what to expect from it's performance and how it will work out it the scale of the .NET framework. Here is a snip: .NET is a "software platform". It's a language-neutral environment for writing programs that can easily and securely interoperate. Rather than targetting a particular hardware/OS combination, programs will instead target ".NET", and will run wherever .NET is implemented.
.NET is also the collective name given to various bits of software built upon the .NET platform. These will be both products (Visual Studio.NET and Windows.NET Server, for instance) and services (like Passport, HailStorm, and so on). I recommend reading this well researched article cause one day or another we all will have to face .NET just like it happened with Windows XP.
I was just checking our logs and quite surprised that more than 88% of our readers already use Windows XP. Read the .NET article @ Ars Technica