JAXP is an API. xml-apis.jar is a jar containing these interfaces.
Have you looked at the website  or mailing list archives ? Usually a good idea to review those when you're new to a project. Lots of answers to questions you may have are already there.
XML Parser Development
IBM Toronto Lab
Richard <firstname.lastname@example.org> wrote on 02/17/2009 03:15:38 AM:
> Hello, I am a newbie.
> I am wondering what's the difference between JAXP and xml-apis?
> Are they the same thing?
> It is shipped in jdk1.4+, why to set up a new project?
> Just for users of jdk 1.3 and earlier?