.NET简介
The .NET Framework (pronounced dot net) is a software framework developed by Microsoft that runs primarily on Microsoft Windows. It includes a large library and provides language interoperability (each language can use code written in other languages) across several programming languages. Programs written for the .NET Framework execute in a software environment (as contrasted tohardware environment), known as the Common Language Runtime (CLR), an application virtual machine that provides services such as security, memory management, and exception handling. The class library and the CLR together constitute the .NET Framework.
From Wikipedia(http://en.wikipedia.org/wiki/.NET_Framework)
它包含一个大的类库，可以提供在不同语言之间的互通性，即一种语言编写的程序可以被其他不同类型的语言识别和使用。
由.NET编写的程序会在一个软件虚拟机中被执行，这个虚拟机就是CLR，它可以提供安全性、内存管理和异常处理等服务，因此运行在CLR上面的程序也被称为托管程序(Managed Program)。
.NET分为两部分：
1: CLR，公共语言运行时
2: .NET framework类库