It looks like that Imperial stormtroopers ARE the regular ground soldiers of the Galactic Empire as was established in 2014 with the debut of the Star War Rebels series.

 

When the original Star Wars RPG came out in 1987, published by West End Games, stormtroopers were portrayed as separate from the regular Imperial Army troopers.

 

The question of Imperial Navy troopers: are the Navy troopers stormtroopers leased out to the Imperial Navy, or are Navy troopers separate from stormtroopers?

Stormtroopers are more iconic than other types, so they play a much bigger role in most representations of Star Wars. Particularly since Disney took over.

 

I always viewed stormtroopers as Imperial marines in the tradition of other sci fi marine corps. They are numerous on Imperial ships and are used in boarding and landing operations. Regular army units are used as garrison forces, and are trained for that assignment. Navy regiments are specialized forces only used on ships or for special assignments where their higher-than-average technical skills are called for.

 

Head cannon aside, regular army units basically don't exist in Disney's Star Wars, and navy troopers are simply operatives or officers of stormtrooper regiments. It simplifies things if you only have one branch of military forces to deal with, and Star Wars has never been into the nitty gritty military details.