Short answers, pulled from the story.
Self-propelled artillery is artillery equipped with its own propulsion system, allowing it to drive to its firing position without being pulled by a separate vehicle. Towed artillery must be stopped, unlimbered, and set up before firing, then re-limbered to move again, whereas self-propelled artillery can stop, fire almost immediately, and leave before a counterattack arrives.
The British Gun Carrier Mark I was the first tracked self-propelled gun fielded in combat, appearing in 1917 during World War I. It was built on the chassis of the British Mark I tank and carried a heavy field gun that could be fired from the vehicle or dismounted.
The Italian 102/35 su SPA 9000 is considered the most produced self-propelled gun of World War I, with more than 100 units built. It entered service in September 1915 and was praised for its ability to quickly reposition itself, supported by more than 7 dedicated support vehicles per gun.
Hauptmann Alfred Becker, a mechanical engineer and battery captain in the 227th Infantry Division, created the first German tracked self-propelled artillery by mounting 10.5 cm leFH 16 howitzers onto captured British Vickers Mk.VI light tanks. His improvised vehicle, designated the 10.5 cm leFH 16 Geschutzwagen Mk VI 736 (e), was the direct forerunner to the German Wespe and Hummel.
The Sturmgeschutz III assault gun was the single most-produced armored fighting vehicle Germany built in World War II. First designed in 1936-1937, it pioneered the casemate-style fully enclosed armor that defined almost all late-war German self-propelled artillery and tank destroyers.
The zamburak was a specialized form of self-propelled artillery from the early modern period, consisting of small swivel guns mounted and fired from the backs of camels. It was used by the Islamic gunpowder empires, particularly those of Iran, where the rugged Iranian plateau made transporting heavy cannons difficult.