Okay, I have been asked this a zillion times now - each time by people who for some reason that escapes me think that *I* would know anything about .NET, what it is, how it works, etc.
I'm not completely ignorant on the matter - I do understand the concepts, architecture, how it all fits together. But I am definfitely not a programmer, and beyond the basics of describing what it is, I can't help much.
In fact, I was at a conference earlier this week along with a bunch of other corporate IT-leader types, and one thing that surprised me was how few knew what .NET is (other than the fact that its something their systems are built on and their developers talk about in gobbledygook secret code language all day long). I suppose the fact that the crowd at the conference was all upper-level and executive management types allowed the people there to come out of their shells a little bit, and to openly ask the people around them what the heck this .NET thing is, anyhow (without fear of some developer rolling eyes, I guess 
).
Now, I always figured it's good to know these things ahead of time, but when some of the Microsofties I know started bringing people over to me so I could explain .NET ... Well, you get the picture.
Of course, what people need is a good, clean, concise explanation of the technology in layman's terms, not programmer-speak, and not more than a couple of minutes. As I recently noted when I wrote about “What is XML?” it's not always what you know, it's more often than not what resources you can find and how you use them.
Anyhow, point is that these days, when I am asked “What the heck is .NET anyhow?” I like to point people to a web page that Microsoft recently put together which - for the first time - provides a very brief, concise and easy to understand overview of the basics of the “What is .NET?” question. Check it out here.
There's more to it than what's on that page, but the links get you to the next level of detail (and they keep it simple at that level, as well, which is good).