MSPX is an extension characteristic of documents used by a collection of Microsoft Internet services called IIS (Internet Information Services). MSPX are files that are used to store websites based on ASP.NET technology. MSPX files are not directly accessible to the web content viewer. They are processed by the web server, which dynamically generates the appropriate HTML file and sends it to the client's browser.

ASP.NET is a framework used to create web applications, developed by Microsoft.