HTTP handler Vs HTTP module :

Http Handlers: 

Http handlers are developed by asp.net developers. It a component which is to implement System.Web.IHttpHandler interface. Handlers is act like ISAPI dlls the only Handlers can be called directly URL while ISAPI Dlls are not.

Process Request:  a processing when asp.net requests in a method. 

IsReusable:  another request fulfill the same type of instance in HTTP Handler.

uses for custom HTTP handlers: 

- To create an RSS feed for a Web site. 

- When users send a request to your site that ASP.NET calls your handler to process the request.

- Image server

Http Modules: 

Http modules are objects which also participate in the asp.net request pipeline but it does job after and before HTTP Handlers complete its work. it will associate a session and cache with asp.net request. It implements System.Web.IHttpModule interface. 

uses for HTTP modules: 

- Security    

- Statistics and logging

- Custom headers