.NET framework 是什么? 有何意义?

通俗的解释：
.NET is a framework that Microsoft has introduced to develop applications easily. A framework can be understood as a collection of Application Programming Interfaces (APIs) and a set of code that a programmer can call without having to write it properly.
.NET是一个微软提供的引用开发框架。框架可以理解为是一套API接口，一个通用代码的集合。有了这套集合，人们可以直接调用，而无需重复编写。

The .NET framework is a repository of huge number of shared code. This set of shared code has made the lives of developers much easier and saved their quality time as they do not write the complete code again and again, instead focus on the code unique to their application.
.NET框架的意义就在于解放程序员，不需要重复写一些相同，重复的代码。而只需要专注于工程内的独特性创新。

Microsoft .NET applications can be written in languages such as C#, F# or Visual Basic. These applications then run on any compatible Operating System. .NET framework supports websites, services and desktop apps, among others.

微软的官方解释：
What is the .NET Framework?
The .NET Framework is a managed execution environment for Windows that provides a variety of services to its running apps.
JRE是JAVA的运行时环境，是保证JAVA程序正常编译、链接、执行的一套工具。如果类比到.NET framework的话，.NET Framework就是Windows的运行时环境。即保证Windows上的apps能够正常运行的一套工具。