Units are the primary unit of organization between you and your soldiers, allowing you to set orders and manage hundreds of individuals soldiers at once. Each unit under your command will have it's own supply source, battle strategy, tactics, name, and deployment settings, allowing you as their leader to maximize their use on the field.
Generally speaking, units are best when homogenous, or made up of soldiers all using relatively the same equipment. Archers with archers, cavalry with cavalry. In battle a unit will only move as slow as it's slowest soldier, and while it can be said that a disorganized mess is hard to fight against, it's also hard to deploy effectively.
For more detailed guidance on how exactly units work in the field, we recommend you read the new battles page.
Gone are the days of First Ones telling the mortals to steal from the locals in order to feed themselves--apparently, even at the behest of a First One they weren't too happy about it--now this is handled entirely under "supply". Supply is either drawn from a settlement under your command, or the settlement of the Unit's origin (TODO). Presently, this is strictly for food, but once a proper method to delay arrival of goods is added, this will also allow field resupply of armor, weapons, and other gear.
Past Supplies, there are two main things you'll concern yourself with in regards to Units: Strategy and Tactics.
Strategy is your unit's course of action when in battle. Specifically, where it will try to orient itself in relation to opposing forces, and from which direction it will attack them. Each strategy is detailed below:
Tactics, on the other hand, is how they will engage the enemy. Will they use ranged weapons? Melee? Or whatever is appropriate? The game will default to "Use Best" if not otherwise declared.
Simply put, these control where your forces deploy in relation to their allies and enemies and whether or not they'll leave fortifications to attack an enemy. They let you fine tune your forces time to encounter as well as make effective use of walls (which give a handy bonus to defenders). This defaults to Line 4, the middle, if not set.
Lastly, while not yet a configurable options, you'll be able to define a unit as militia or not.
Militia units will contribute to their home settlement's economy while there and will be able to bolster their numbers significanlty faster. Their training may include some basic weapon usage, but in times of emergency (like when under siege) it may just be giving them whatever weapons are available and hoping for the best. Lastly, they take penalties when going too far afield. You can march them across the continent, but it's not what they "signed up for".
Professional Units will trade that off for a passive experience gain and combat effectiveness bonus, being more likely to hold together in combat, and less likely to retreat when a fellow unit breaks or a First One falls in combat on their side. Their bonuses bolster both attack and defense, as well as maintaining morale and resisting opportunities to break and run. They're also the only forces capable of operating siege equipment afield.