Smoke comes in two forms, direct and bombardment – and it has two principle effects – units shooting through smoke can only see 16 inches, and if the target you’re shooting at is obscured by smoke it counts as concealed and gone to ground – except for defensive fire where it’s just concealed.
However there are some difficulties with smoke, you have to fire it at an enemy unit – so still need to roll to hit. Bombardments spread according to the wind direction – a random roll on the first smoke bombardment. Smoke bombardments must be the first shooting in the shooting step, and direct fire smoke is resolved before normal shooting from a platoon. Despite all of that smoke is key to FoW.
With the exception of soviets pretty much every faction has access to this, it can range from cheap mortars and nebelwerfers to large artillery pieces. On offence the main aim early on is to use smoke to stop the enemy shooting you. Now you move then fire smoke so a bad dice roll can leave you exposed, but frequently if you drop smoke on an enemy unit they have to decide to either not shoot you, (over 16 inches away) shoot you at a +2, or move out the smoke and drop their RoF.
On defence smoke bombardments are great vs enemy spotters (stop their bombardment) or to protect one of your defensive units which will be shooting, from an opposing unit which can hurt it.
But the third important area is combat, frequently you will have to assault a unit in the open, being able to assault through smoke makes you harder to hit from defensive fire – but almost as importantly, after combat it gives you somewhere safe to consolidate into.
The classic units which provide this are mortars and shermans etc. Direct smoke is a powerful tool, but requires huge discipline to go to reduce enemy effectiveness vs damage.
There are three principle ways I use it. The first, like the bombardment, is to just obscure an enemy unit. This is a great tactic when faced by some giant beast like a Jagdtiger! The other options though require synergy with other units. The first is sniping. Take this example, my Brits are facing soviets, the Shermans fire direct smoke, unlike normal shooting I choose where it goes, and I place it to obscure every tank except the command. I then use my M10’s to fire, the shots first are resolved against the easiest to hit tanks – e.g the platoon commander. He’s taken out and the platoon needs a new commander to move! A very effective tactic.
The other is on defence, if you have AT guns and mortars for example , the AT can shoot the enemy tanks, which the mortars can then direct smoke. Again this leads to the decision to stay still and shoot at +2 or move and drop RoF
I hope this has helped understand smoke, if you have any other great tactics let me know, or if you want me to cover areas drop me a line too.