Same thing as a TaskDatabase, only that this database only contains bugs (defects).
A TaskDatabase is preferred to a BugDatabase because it has more information and it relates all the information. A BugDatabase only can relate bugs.
Scrum advocates to have a TaskDatabase. ExtremeProgramming does not advocate to have a BugDatabase, much less a TaskDatabase.
We just call everything a bug and let it at that.
See also: TrackingTool, BugTrackingSoftware.