It is called Order of Battle (OOB) and here is the general breakdown from top to bottom.  It also only generally holds true for Western Style forces.  You get away from Western forces and Order of Battle gets crazy.  The Russians and Chinese use a roughly similar but at the same time totally different system with totally different names for some echelons.Army and Marines(minus Armies for Marines):  Army Group, Army, Corps, Division, Brigade, Battalion/Squadron, Company/Troop/Battery, Platoon, Section, Squad, Fire TeamAir Force:  Air Force, Wing, Squadron, Flight, SectionNavy: Fleet, Task Force, Battle Group, Division, VesselOf course, there is also an org chart within vessels but that is different for every vessel.

Heck, the only reason I know so much about how the American army is organized is because I spent 23 years in, otherwise I would know way more about the Prussian Army than the American.OOB is only roughly equivalent within armies over time.  The Regiment used to be the mainstay of the US army but it is now the brigade.  The only unit I kno of that still has a regimental org. in the US Army are the 3 cavalry regiments 2nd, 3rd, and 11th US Cavalry and the 3rd Infantry (Old Guard) that guard the Tomb of the Unknowns at Arlington.