An ASP.NET HTTP handler is a process that runs in response to a request made to an ASP.NET Web application. The most common handler is the ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page through the page handler.

HTTP handlers are an essential component of the ASP.NET framework, providing a low-level way to interact with incoming HTTP requests. They offer developers fine-grained control over how specific requests are processed, allowing for custom handling of various file types or URL patterns.

HTTP handlers were not present in the "Classic" ASP. They implement the System.Web.IHttpHandler interface. Unlike ASP.NET Web Forms, they have no HTML-markup file, no events and other supporting. All they have is a code-file (written in any .NET-compatible language) that writes some data to the server HTTP response. HTTP handlers are similar to ISAPI extensions.

The IHttpHandler interface defines two key members:

Developers can create custom HTTP handlers to implement specialized functionality, such as:

An HTTP module is an assembly that is called on every request that is made to your application. HTTP modules are called as part of the ASP.NET request pipeline and have access to life-cycle events throughout the request. HTTP modules let you examine incoming and outgoing requests and take action based on the request.

While HTTP handlers are responsible for generating the response to a specific request, HTTP modules can intercept and process all requests that pass through the ASP.NET pipeline. This makes modules ideal for implementing cross-cutting concerns such as logging, security, or performance monitoring.

Unlike ASP.NET Web Forms, that have ".aspx" file extension, ASP.NET handlers by default have ".ashx" file extension.