通俗的解釋：
.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是一個微軟提供的引用開發框架。框架可以理解爲是一套API接口，一個通用代碼的集合。有了這套集合，人們可以直接調用，而無需重複編寫。
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框架的意義就在於解放程序員，不需要重複寫一些相同，重複的代碼。而只需要專注於工程內的獨特性創新。
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.
微軟的官方解釋：
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的運行時環境，是保證JAVA程序正常編譯、鏈接、執行的一套工具。如果類比到.NET framework的話，.NET Framework就是Windows的運行時環境。即保證Windows上的apps能夠正常運行的一套工具。