ASP is a server side scripting framework for making dynamic web pages. For all intents and purposes it is a programming language, though the actual underlying language is C#, VBScript or similar. Microsoft created ASP and still owns it. For various reasons, we use PHP instead. Some of these reasons include:

• ASP is a closed source language, meaning only Microsoft employees know it’s inner workings.
• It only works well on Microsoft server software. It can be made to work on other server software but this is apparently a painful endeavour.
• It chews through memory in web servers it is deployed on. An equivalent program written in PHP can run on less powerful hardware.

ASP.NET Resources

Read more about ASP at this Orient Software article comparing ASP and PHP and the ASP.NET Wikipedia article.