What is Stuff in GTD?
It is anything that pops up into your world that does not belong where it is, and you have not yet defined what exactly it is. It is still something amorphous, which needs to be clarified.
Stuff differs from incompletes in that incompletes are already defined (you know what they are), and what may be to be determined is the next action you should do with them; or this might be decided as well and you just have to do it and take it to completion. Sometimes it is just about realizing that you don't have to do anything else with it.
Basically, everything you capture is stuff.