What can & can't be programmed with .NET?

Question

Hi guys,
 
I'm studying programming with C# and .NET at the moment. I was wondering if anyone could tell me whether or not .NET could be used to program applications for non-Microsoft operating systems such as Google Android or Chrome and also whether .NET could be used to program apps for things like the Apple iPhone?

Also, if this is the case, is .NET a good platform to create apps for these non-Microsoft operating systems, or is it that maybe it can be used in this manner but really isn't the best way to program for these operating systems or the best way to use .NET?

PS. All answers are welcome : )

Thanks very much indeed,

Dan.