Backbone of your army (duh). Composition depends entirely on who you are facing, who you are playing as and what's your industry like. On average, you want lots of infantry with some artillery to it, potentially also AT artillery>Tank
A power multiplier for your mobile units, used to power-through enemy frontline and create encirclements, while not being shat to pieces. You know, breakthrough the front line and flow through the gap with other units. NEVER make pure tank divisions, they MUST come with other mobile units, like trucks..>Motor
Can either go on their own or as combined arms component to your tanks. Their main appeal is speed, while combat wise, they are slightly better than foot infantry. Still, even if you don't have tanks, you want to have at least a token force of motor divisions (one of DLCs adds motor artillery, so throw that in, too) and use them to overrun enemy positions>Fighter
Air support for your fighting troops, essentially another power-modifier. Particularly good at killing tanks and mobile units.>Naval bombers
Unless you plan to do naval invasion or fighting a naval power - don't bother>Interceptor
As the name implies, intercept enemy bombers and defend your units and installations from being bombarded. You can ignore pretty much every other element of air combat, except for those guys>Strategic bomber
Don't bother>Tatical bomber
Don't bother. Some might argue that they are long-range CAS, but they are "jack of all trades, master of none" waste of resources and production.
On average, you want to have 9:1 ratio of infantry to motor divisions and if you can afford tanks, then 6:2:2 of infantry, motor and tanks.
And next time, read a fucking game manual before asking.