Refer the article in below link: http://msdn.microsoft.com/en-us/library/bb398986.aspx

HTTP Handler and HTTP Modules are basically used as request handlers in the ASP.Net applications. Both are provided with low level access to requests and responses. HTTP Handlers and Modules are registered with an application in the configuration file under httpHandlers and httpModules section respectively. HTTP Handler is used to process request made for a file with a certain extension, path or request type whereas HTTP Module enables you to participate in the processing of every request. The ASP.Net Framework includes several standard Modules for managing state and implementating Authentication schemes. For example - the output cache, session state, forms authentication and windows authentication are all implemented as Modules.