Okay, for the Canadian Army (which was modelled off of the British Army) you have a number of different types of regiments; right now, I'll get you the Canadian Infantry Regiment since it's near midnight where I am, and tomorrow evening I'll get to the Motorized Infantry Regiment, the Armoured Regiment, the Artillery Regiment, and the Armoured Car Regiment:

Infantry organization:

Section - a 10 man unit, divided into a gun group (the Bren gunner, plus one man with extra ammo and equipment and a Lance Corporal or Corporal for command purposes) and a rifle group (6 riflemen, plus a section leader, usually a Corporal or Master Corporal) whose purpose it was to manoeuvre toward the enemy while the Bren gun provided covering fire.

Platoon - three sections, plus a weapons group of one two-man Boys Anti-Tank Rifle or, later, PIAT anti-tank projector team, and one two-man 2-inch mortar team, a Platoon commander (usually a Lieutenant,) a Platoon 2IC (a senior Sergeant or Warrant Officer,) and a batman/runner/signalman. The command group would, obviously, coordinate the platoon in attack and defence, while the weapons teams would provide support (the 2-inch mortar was particularly useful for laying down smoke cover.)

Rifle Company - three rifle platoons made a company, which was commanded by a Captain or a Major, along with their attendant staff for communications, logistics, etc.

Support Company - Canadian and British Infantry regiments had a company dedicated to providing support weapons; it was organized similarly to the Rifle Company, but the constituent platoons were different. It contained the following sub-units:

1 Mortar Platoon consisting of three sections of two, three-man 3-inch mortar teams, plus a command and observation team and, (optimally) a Universal Carrier and PIAT/Anti-Tank Rifle team. Commanded by a Platoon Command team organized similarly to a Rifle Platoon.

1 Carrier Platoon of 13 Universal/Bren Carriers separated into three sections of four Carriers, plus a command carrier (they would be equipped with Bren guns, mortars, and anti-tank rifles or PIATs, as well as bicycles to provide rapid communication, and were usually used as scouting or harassing units)

1 Anti-tank Platoon of three, two-gun 6-pdr Anti-tank gun teams, along with their Universal or T16 or Loyd carriers, and a platoon command team to provide coordination and communications.

1 Pioneer Platoon of engineers and sappers, who provided not only standard combat engineering support (repairing bridges, clearing minefields, etc.) but also acted as assault teams using flamethrowers when they were issued.

Headquarters Company: This consisted of two platoons of Administrative and Signals staff, and was typically commanded by a senior Major.

Battalion: Four Rifle Companies, a Support Company, and a Headquarters Company formed a Battalion, which was commanded by a Lieutenant Colonel, with a senior Major as 2IC, and contained medical, intelligence, security, and provost/military police sections, as well as a section of orderlies to carry out the day-to-day administration.

Regiments: Canada, the UK, and the Commonwealth differed from the Americans in that we organize our Battalions into Regiments; in peacetime, a Battalion is a Regiment, generally speaking, though during WWII, most Regiments fielded several Battalions which would be referred to as X Bn, "Regt. name." They were administrative entities only.

Brigade: Three Infantry Regiments plus a "ground defence platoon" of infantry would form an Infantry Brigade, which was in turn commanded by a Brigadier General.

Infantry Divisions were made of a Divisional Headquarters consisting of the Divisional Command (a Major General,) a Machine Gun Regiment, a Reconnaissance Regiment, and an Armoured Car Regiment, plus three Infantry Brigades, as well as Divisional Artillery (consisting of three Field Regiments, an Anti-Tank Regiment, and an Anti-Aircraft Regiment,) a Regiment of the Royal Canadian Engineers, and units from the Royal Canadian Signal Corps, Medical Corps, Army Service Corps, Ordnance Corps, Electrical and Mechanical Engineers, Postal Corps, Provost Corps, and Intelligence Corps.

Corps were groupings of Divisions, usually three, plus additional Brigades and Divisional Troops (individual Regiments that fulfilled specific roles in the Division, such as liaising between units, providing reconnaissance, intelligence, etc.) and led by a Lieutenant General. The composition of a Corps was fluid:

I Canadian Corps contained, 1st Canadian Infantry Division, 5th Canadian Armoured Division, and 1st Canadian Armoured Brigade, plus Divisional Troops

II Canadian Corps contained 2nd Canadian Infantry Division, 3rd Canadian Infantry Division, 4th Canadian (Armoured) Division, 2nd Canadian Armoured Brigade, Divisional Troops, and at various times between 1944 and 1945, 1st Polish Armoured Division, 15th (Scottish) Infantry Division, 51st (Highland) Infantry Division, 33rd Armoured Brigade, 1st Belgian Infantry Brigade, the Royal Netherlands Motorized Infantry Brigade, and 1st Czechoslovak Armoured Brigade.

Finally, an Army was a grouping of one or more Corps, plus administrative, artillery, support, armoured, and infantry elements that were required for specific tasks at Corps, Division, or even Brigade level, and was commanded by a General.

Army Groups were collections of Armies, and were commanded by either senior Generals or, in the case of 21st Army Group (where 1st Canadian Army was assigned) a Field Marshall. Field Marshalls were the senior ranking officer in a nation's army, and all other Generals were subordinate to their command - for obvious reasons, nations only had a single Field Marshall (or, in the US, 5-Star General) at any given time.

It's a lot to take in, but I hope it helps!

The Wehrmacht's OOB was,

• OKW (Ober Kommando Der Wehrmacht), the head of this was ostensibly the German Chief of Staff, but for all purposes this was headed by Hitler directly.

• Subordinate to OKW, but not directly reporting to the OKW was the OKH - Ober Kommando Der Heere, and the individual Naval and Airforce commands.

You then had the operational groups,

• Army Groups - Headed by Field Marshals. Each AG had anything between 2 to 4 infantry armies and a Panzer army or two.

• Armies - Usually comprised of anything between 2 to 4 corps and headed mostly by a Colonel General ranks

• Corps - As above, comprised of anything between 2 to 4 divisions and headed by full Generals

• Divisions - Comprised of anything from 2 to 4 regiments and headed by mostly Lt Generals

Beyond this it gets too granular, so will stop right here.

Very broadly, the differences in ranks are the scope and scale of their command. A FM will head an entire army group while a general would run only a corps. Their roles and responsibilities were also different as Army Group HQ was deep into staff work while a line command like a Corps command involved a lot of staff work as well as more hands on approaches to directing combat.