What is an HTTP handler in ASP.NET? Can we use it to upload files? What is HttpModule?

The HttpHandler and HttpModule are used by ASP.NET to handle requests. Whenever the IIS Server recieves a request, it looks for an ISAPI filter that is capable of handling web requests. In ASP.NET, this is done by aspnet_isapi.dll. Same kind of process happens when an ASP.NET page is trigerred. It looks for HttpHandler in the web.config files for any request setting. As in machine.config default setting, the .aspx files are mapped to PageHandlerFactory, and the .asmx files are mapped to the WebServiceHandlerFactory. There are many requests processed by ASP.NET in this cycle, like BeginRequest, AuthenticateRequest, AuthorizeRequest, AcquireRequestState, ResolveRequestCache, Page Constructor, PreRequestHandlerExecute, Page.Init, Page.Load, PostRequestHandlerExecute, ReleaseRequestState, UpdateRequestCache, EndRequest, PreSendRequestHeaders, PreSendRequestContent.

Yes, the HttpHandler may be used to upload files.

HttpModules are components of .NET that implement the System.Web.IHttpModule interface. These components register for some events and are then invoked during the request processing. It implements the Init and the Dispose methods. HttpModules has events like AcquireRequestState, AuthenticateRequest, AuthorizeRequest, BeginRequest, Disposed , EndRequest, Error, PostRequestHandlerExecute, PreRequestHandlerExecute, PreSendRequestHeaders, ReleaseRequestState, ResolveRequestCache, UpdateRequestCache