What's .NET?

Purpose:

• This question checks whether a developer has a understanding of the actual engine under the hood
• Shows that they are not Code-by-Google type of developers
• Tests whether the developer is keeping abreast with developments in the .Net ecosystem, inability to answer may indicate stagnation or ignorance.

Potential Answers:
.NET Framework is a framework developed by Microsoft. Traditionally it used to only target Windows operating systems but industry trends like containerization has led to Microsoft backed open source version of the framework (.Net Core) being developed that can target other platforms. (If the developer starts talking about .Net Standard and the purpose that is serves it is a good sign)

.NET includes a Framework Class Library (FCL) and provides language interoperability across several programming languages (languages can interact with each other and share the FCL).

.Net programs run in a software environment named Common Language Runtime (CLR), an application virtual machine that provides services such as security, memory management, and exception handling. FCL and CLR together constitute .NET Framework. (If the developer starts talking about the Common Language Specification and Common Type system it is a good sign)

At the time of this writing there are numerous Language Runtimes (A warning sign would be if the developer has made no effort to get to known other environments beyond traditional .Net framework):

• NET Framework - Traditional .Net framework developed by Microsoft. Closed source and targeting Windows OS. 
• Net Core - Open sourced version of .Net, can target Windows, Linux and Mac, plays nicely with containers. 
• Mono - Original industry open source port of .Net. Mono for Xamarin is also important for brining .Net to iOS and Android