There are three categories of units- ground, air, and navy (the latter of which is usually kept in fleets). Each need to be utilised to their full potential in order for the best outcome. The descriptions here are simple, click the article headings for further details.
Ground Units
Airbases launch fighters and bombers- they can be used to scout, engage the enemy, or (in the case of the latter) launch nuclear missiles.
Missile silos can be used to either shoot down enemy nuclear missiles or launch ICBMs to enemy targets.
RADAR is used to see the assets of nearby powers and incoming attacks.
Air Units
Bombers can be launched from both airbases and carriers. They are used mainly for launching nuclear missiles, but can also be used for scouting due to their long range.
Fighters can be launched from airbases and carriers. They can be used for scouting nearby enemies and for engaging enemy ships and aircraft.
Battleships are used to attack enemy planes and ships, but not submarines.
Carriers are used to launce fighters and bombers to take on enemy units. They can also be used to destroy enemy submarines.
Submarines are equipped with five nuclear missiles to bomb enemy targets. While submerged they can only be seen by carriers, however when they surface to fire they can be seen by all other units.