在几个项目中都有看到这个东西，也零零碎碎读过各种介绍，其实都没有一个很好的概念。StackOverflow 上面有一个解释挺好。记录一下。
Q：What is the most important use of AppDomains in C#?
A:The single most important use is that your code has to have one - i.e. everything you write in C# executes in an AppDomain. That is quite important ;-p
If you mean additional app-domains:
When using plugins and other untrusted code, it allows you both isolation, and the ability to unload them (you can‘t unload assemblies - only entire app-domains).
I‘m using it currently to load dynamically generated dlls, so that I can unload them.
They also allow you to set different configuration files, trust levels, etc - but have associated costs of complexity and remoting.
MSDN has a section on app-domains, here.
原文：http://www.cnblogs.com/ssjzhong/p/4601840.html