HTTP modules are called as part of the ASP.NET request pipeline and have access to life cycle events throughout the request.
An   HTTP   module   is   an   assembly   that   is   called   on   every   request   made   to   your   application.   HTTP   modules   are   called   as   part   of   the   ASP.NET   request   pipeline   and   have   access   to   life   cycle   events   throughout   the   request.（这里怎么理解呢？）  
我不是问英文怎么解释啊，
是问，HTTP   modules   are   called   as   part   of   the   ASP.NET   request   pipeline   and   have   access   to   life   cycle   events   throughout   the   request.
这个东西在实际操作中是怎么样一个过程，
举个例子也可以，
谢谢

------解决方案--------------------
就是每个请求都要经过httpmodule的管理，负责处理整个应用程序周期。

这个的使用有很多的例子，比如统一的权限验证，异常处理，全站的主题设置等。

基本上你需要全局处理的东西，写到这个里面会很方便。