Ground troops serve to keep you own population in line, i.e. they counter unrest if your colony is overcrowded, or the colonists feel, there is not enough of a fleet precense to keep ´em save.
Of course, the main mission of your ground forces is taking and holding enemy real estate.
You drop them on an enemy colony and start to battle the enemy ground forces. Once all defending ground troops are taken care off, the enemy will surrender.
Now you need troops to keep the enemy population from revolting again and accepting, grudgingly, your rule. As your occupation continues, the enemy people become accustomed to your ruling and their status will improve.
As a general rule:
Organize your units (battalions) into brigades by assigning 4 battalions to a brigade HQ.
Organize your brigades into divisions, by assigning 4 brigades to a divisional HQ
Assign ground leaders with a healthy ground combat bonus to your combat battalions, brigade HQs and divisional HQs
Put those leaders with both a combat bonus _and_ a ground force training bonus in your HQs
Note: those boni stack, which means, you can increase the combat efficiency of your units a great deal by choosing the right leaders
For example, let´s say, your heavy armor battalion has a combat value of 16, now you assign a colonel with a 20% combat bonus to that battalion, put it into a brigade with a 10% combat bonus and have that brigade in a division with a +20% leader. This will bring the effective combat value of that battalion to 24 (50% total bonus)
Disclaimer: I am not sure if the various boni will be added first and the applied, or if they are applied one at a time. It realy doesn´t matter. It is a great benifit either way.
For conquest, heavy armor and assault infantry are best (attack rating are what counts here)
For occupation, mobile infantry and garrisons are best, as the defense value is important.
Troop transport modules serve to carry your troops where you want them to be. Be aware that loading/unloading takes quite some time, so those are not the best choise for an actual invasion.
Troop drop modules are what should be used in an opposed landing, as they unload _way_ faster (basicly, they are the equivalent of an air-drop compared to unloading on the pier at a leasurly pace)
There is a drawback to those, however. As the troops are loaded in a "ready to drop" status, staying in them for any length of time is very _very_ exhausting which will impact moral and combat efficiency after a couple of hours/days.
Many, me included, have gone over to pair actual troop carriers with dropships. Ferrying the troops to the hostile system in the TTs and loading them over to the dropships short of the enemy planet to invade.
