HTTP modules and HTTP handlers are main components of the ASP.NET architecture.When ever a client make a http request to the web server, this request flow through a series of http modules and is handed over to http handlers. Http handlers process the request and the response passes through the all Http modules agan in reverse order.
It's just like a filter. The Modules are called before and after the handler executes.
For example: BeginRequest, AuthenticationRequest event, EndRequest event etc. You may intercept , participate and modify each request.
Modules implement the IHttpMudule interface located in the System.Web.System.
You may think it as a program.(or handler, or module), it execute some code when the user send some request. An .aspx page can be thought as a HTTPHandler too, which implements more functions.