A motorized field artillery or anti-tank gun. The difference between it and a tank is that the latter is a versatile support vehicle designed to help infantry, while a self-propelled gun is a mobile gun with a single purpose. A tank destroyer is technically a self-propelled gun but because of the similarity to a normal tank, they tend to be grouped under tanks.
Anti-tank self-propelled guns generally have the gun mounted to the chassis itself in a turretless design (like the German Stug/Stuh or Russian SU-series of guns). Artillery pieces are more varied in design and can have a turret or an exposed cannon.
This only applies to artillery pieces with a cannon; mobile rocket launchers are not covered by this term.
