HTTP modules are a method for interacting with the request. Custom modules must implement the System.Web.IHttpModule interface. Modules are typically synchronized with events of the System.Web.IHttpModule class (implemented within the Global.asax.cs or .vb file). HTTP modules can be used to get various statics like which ip is accesing the page. It can be used for url redirection. Its a mediator between client and server.

An HttpModule is an assembly that implements the IHttpModule interface and handles events. ASP.NET includes a set of HttpModules that can be used by your application. For example, the SessionStateModule is provided by ASP.NET to supply session state services to an application. Custom HttpModules can be created to respond to either ASP.NET events or user events.