an API (application program interface) is a method that a programmer writing an application program can make requests of the operating system or another application.
If I'm not mistaken, I think you might be in the wrong section of the forum. This forum is for a Publishing PROGRAM called QuarkXpress, not a "quark".
There are xtensions in QuarkXpress however for API files. Is that what you were referring to? So far to my knowledge, there isnt a website that contains a LIST of ALL the API xtensions for Quark. The only way to obtain them is to first decide which xtension you need, then research into that and download from their specific websites. Hope this helps.