Is asp.net used for web development only?
For ASP.Net - yes. For the language(s) and the .NET platform that often are used with them such as c# and vb.net, no.
You could be using Web Services as one of the pieces of the whole solution being built/deployed.
So, I say no if you mean only to develop sites.
Yes ASP.NET is for web development such as PHP is made for web development too both are interpreted programming langauges.
ASP.NET has less benefits as it can only be ran on windows operating systems hence why it's lessor use, as most hosts run on a linux os.
Well they made a new interpreter called Mono which allows you to run ASP.NET on linux but its longer to implement in the long run. As well as most cheap hosts will not allow you to install Mono and if you pick windows host it will be slightly more expensive.
Yet PHP is portable and made for mostly all operating systems including linux, windows and Mac OS
Doesn't mean you cannot use PHP to make executable applications without website interface, all thats really required is to pass your script onto the interpreter and it will give out a output for both ASP or PHP