The nature of the impactor is presently unknown. It might have been an asteroid or a comet. In either case, a dark and cindery debris field is expected to develop around the impact point; that's what has happened in the aftermath of previous Jupiter impacts. Professional and amateur astronomers are encouraged to monitor Jupiter in the hours ahead, and stay tuned for updates.
Omomyid writes: Space Weather has the story of a bright impact observed on Jupiter, they also have video (WMV file) from one of the amateur sky watchers — pay attention, it goes by quick! From the article: