This is an extremely short prequel to the Harry Potter novels. A little mini-adventure - a glimpse into the exploits of young James Potter and Sirius Black.
It's not really long enough to be termed a short story (in my opinion), i'd call it an 'excerpt', but of course its not excerpted out of anything, and never will be because JK won't write any prequel novels.
:( <-- thats my sadface.
I did read in a recent article however, that she will 'never say never' to Harry Potter sequels tho, which is good to know!