Aircraft are aerial units. They are good for weakening ground units before you send your army and they can eliminate other aircraft too. They also can reduce the output of cities and weaken your enemy's Economic income and Treasury, or be used for transporting troops. There are four types of aircraft, the Attacker, Bomber, Fighter and Transport Aircraft.
Attacker
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Attackers are effective against ground units. They are similar to artillery, although with a smaller upkeep and much higher damage output.
Bomber
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Bombers are aircraft that bomb enemy cities. They deal a decent amount of damage to enemy troops as well. They have a large amount of endurance, however are far too slow to escape fighters. They can bomb cities and reduce their tax, manpower, and resource output, making them good for crippling the enemy's Money. If your bombers attack an enemy city long enough, it will scorch the city. Scorching an Enemy's capital city using bombers is a good idea early in the battle. Be careful of enemy Anti-Aircraft units, as they can shoot down your aircraft in a matter of seconds.
Fighter
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Fighters are the most effective against other aircraft. They strike down enemy aircraft, and they enjoy faster mobility compared to attackers and bombers.
Transport Aircraft
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Transport Aircraft are units which are able to transport 1 division of any ground unit (e.g. 10k infantry, 200 tanks). They're best used for capital sniping or transporting troops across harsh terrain/biomes, such as mountainous terrains and arctic biomes, in order to avoid attrition or simply get them across faster. They are extremely vulnerable to other aircraft, however, specially Fighters.
Aircraft Requirements
Once away from an airport or an Aircraft Carrier, aircraft will begin losing endurance. When their endurance is depleted, they will fly to the nearest of your or your ally's airports to regain endurance. Also, they require a considerable amount of money and Manpower, as well as 1 Aircraft Part.