Enemy Units
Gliders
Shoot them. They will target anything that shoots at them first. They can be destroyed by bombers from your airport.
Helicopters
Fear them. Helicopters have a longer range than lasers but not turrets. They are also slower than Gliders so they arrive at your base late after you think the wave is done. Zooming out during a wave can help you spot an incoming helicopter for early detection and to prepare for it by building necessary turrets near where it will attack.
"Sad Robots*"
Run and hide. Once destroyed they deploy a squadron of gliders. Fun. Some levels require you to bomb them and destroy the gliders they release at the same time. To do this, designate 2 or more bombs to be dropped in quick succession on the Sad Robot or else a stray glider may escape and most likely destroy your base. If possible try to drop 4 bombs to maximize chances of success.
*I initially used the name "Walking Death-Things" but yiotro uses the much more adorable "Sad Robots"
Drones
These ones are fun. Airplanes forever. Drones fly over your entire base and plant bombs. Use carrier units to move the bombs to an airport or junkyard. Sometimes multiple junkyards with each set to accept and burn a low quantity of bombs is the fastest way to clean them from your base before the next enemy attack and destroy a module with a bomb still in it.
Enemy Bases
They're like cancer, kill them early. A base can grow from any type of enemy module. Typically they grow in the direction of your base.
A strategy that can be employed is bombing or destroying the majority of the base but leaving 1 spawner left alive. This is useful where the tasks to complete the level include both surviving waves and destroying the enemy base. If the base is destroyed before all of the waves are completed, new enemies from a new wave will spawn at the edge of the map in random locations, whereas if 1 spawner of the enemy base is left alive, all enemies that spawn from a wave will come from that spawner. This makes defending easier because defensive modules can be placed in the direction of this last spawner.
Platform
From which their base grows.
Spawner
Produces enemy gliders and drones.
Machine Gun
Enemy machine guns have a range longer than lasers, but shorter than turrets. Does a high amount of damage.
Enemy Behavior
"Enemy" pretty much describes it.
Some maps have gliders already spawned where they just fly around in a small area only leaving the area if attacked.