The extension ASP stands for Active Server Pages.
Because ASP files are typically run on a web server, any browser can be used to display the resulting document.
asp files are similar to PHP or CGI. The users never get to see the actual asp file, but only the created HTML page.
All you need to do to develop ASP pages is a text editor. Preferably, an editor that recognizes the language elements of ASP, color-coded and structured represents. There are also special applications that generate ASP code for specific applications, eg database connections.
Microsoft has discontinued the development of ASP. ASP.NET is the successor technology.
However, this uses the extension aspx.
The extension ASP is mostly used instead of htm or html on websites, if the server is running under Microsoft IIS or uses ASP.NET. There are also various extensions for other web server systems that allow you to execute ASP code.