ASP (Active Server Pages) which was also commonly referred as Classic ASP is Microsoft's first Server side scripting engine to dynamically generate web pages.
ASPX - In 2002 Microsoft released the successor to ASP called ASP.NET.
Extract from Wikipedia:
The initial prototype was called "XSP"; Guthrie explained in a 2007 interview that, "People would always ask what the X stood for. At the time it really didn't stand for anything. XML started with that; XSLT started with that. Everything cool seemed to start with an X, so that's what we originally named it."
The initial prototype of XSP was done using Java, but it was soon decided to build the new platform on top of the Common Language Runtime (CLR), as it offered an object-oriented programming environment, garbage collection and other features that were seen as desirable features that Microsoft's Component Object Model platform did not support. Guthrie described this decision as a "huge risk", as the success of their new web development platform would be tied to the success of the CLR, which, like XSP, was still in the early stages of development, so much so that the XSP team was the first team at Microsoft to target the CLR.
Once the ".NET" branding was decided on in the second half of 2000, it was decided to rename ASP+ to ASP.NET.
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