Both. Stormtroopers specifically refers to special units of the German Army post 1916, who were trained specifically for assaulting and seizing enemy trenches in the beginning of an attack. Stromtroop tactics were tactics used by these units but could also refer to trench raiders who were employed by all major nations during the War.

By the end of 1916 the armies on the Western Front were locked in trench warfare and unable to overcome one another's defense. The large mobile armies of the past could not break through and advance fast enough to achieve success on the battlefield. However by this time the nature of trenches themselves had changed. Whereas early trenches were simply lines fully manned by riflemen. Heavy artillery barrages caused large numbers of unnecessary casualties in those packed trenches. Forward trenches gave way to interlocking strong points manned by machine gun teams, who could provide supporting fire while minimizing the number of soldiers actually present in forward trenches.

These more isolated defense were vulnerable to, and attacked by, smaller groups of infantry who would sneak across no-mans land and conduct a localized attack. They would use grenades, surprise, and close quarters weapons (pick axes, knives, clubs, or spades) and would quickly assault a trench usually to capture prisoners and material, or eliminate a particularly valuable machine gun position for example. Instead of holding the ground like in a normal attack, they would withdraw before the main reserve force of the enemy defenders arrived.

Stormtroop tactics then, were these trench raids made official doctrine. Although all armies experimented with special forces, trench raiders, and the like. The Germans were the only army to build actual purpose defined and trained Stormtroop units, called a Sturmbattalions the first of which appeared in March of 1916. These new units represented a synthesis of ideas from all over the German battlefields including the eastern front. They key advancements were increased training, motivated volunteers, greater leadership delegation, and more effective coordination between the small units of front line stosstruppen and heavier backup supporting arms. Later stormtrooper units adopted their own supporting arms, such as trench mortars, and attacks focused on combining heavy supporting fires with small pinpoint attacks, followed up by a strong reserve of regular infantry to hold the new ground. Any army using these types of maneuvers can be said to utilize Stormtroop tactics. Indeed these kinds of infiltration and assault tactics are still taught today to modern armies.