[Gravdigr]

From Quora:

Tank - an armored fighting vehicle armed with a high-caliber cannon on a rotating turret designed for direct engagements against other armored fighting vehicles.

Self-propelled artillery - an artillery platform with its own propulsion system. They are equipped with guns of a usually higher caliber than tanks (most have around 155 mm. cannons compared to the standard 120-125 mm. guns used on tanks) but are positioned on a higher angle for bombardment of long[er]-range targets. Also, unlike tanks, the armor of an SPA is usually only effective against small-arms fire and shrapnel since they are not designed for frontal engagements.

Self-propelled guns - also known as assault guns or tank destroyers. They're used for direct infantry support destroying fortifications and defensive emplacements with high-explosive shells. Can also be used either to take down other armored vehicles or as "short to medium ranged" artillery in some cases. Self-propelled guns are built around a forward-facing casemate placed on top of a tank chassis forcing the entire vehicle to turn around or rotate if facing a target in another direction.

FWIW