So that's
basically all you need to know about cities; now let's look at units.
Units have all the expected stats and then some.
HPThe green vertical bar with a number atop is their HP; this can be recovered by spells, a handful of traits, or by resting when there's no adjacent enemies. HP never increases unless you upgrade the unit to its more powerful form, so watch out for that.
Attack, Type, and RangeMoving on, we've got a purple arrow thing with a number and then a curving arrow and another number. That's it's attack damage (main number), damage type (arrow, in this case Missile), and range (curvy arrow, not shown for melee units). Units with additional damage types, as is fairly common, will have additional icons above their damage bar, and a more thorough breakdown can be had by mousing over it.
Movement and TypeThe boot-and-bars thing below that looks self-explanatory at first glance, but does have a few nuances to it. Specifically, movement is heavily affected by terrain, but some units have traits or enchantments to alleviate that. The foot icon is actually different for flying or swimming units, incidentally. Open terrain like plains costs 1 point of movement per tile, hills or forests 2, and swamps can cost three. This can make moving through hills with a 3-speed unit rather torturous, but at least they can be auto'd to move over several turns and thus not lose their fractional moves.
Experience and LevelBelow that we've got their experience bar and level. Units gain 1 exp per turn just from existing (though not if they're on a boat, sadly), but since it costs 20 exp to gain level 2 and keeps ramping up from there, just sort of stashing a unit in the corner is of limited utility in leveling them up. More useful is actual combat- units gain (I think) 5 exp for attacking in melee and 4 exp for anything else, with the strength of the opponent being irrelevant.
PerksNow, if we'll jump down a bit to those little boxes, one with an eye and one with a tree, we'll talk about the benefits of leveling. Each level, units get to pick from three traits; these traits are predefined, but I assume it modifies your options when you've already taken one of the three it was going to offer that level. Traits show up as little boxes like those; in this case, those are innate traits, but leveled ones look pretty much the same. Enchantments or gear upgrades also go there.
ClassIf we move back up to their icon, we see a little bow and arrow icon inset into it. That signifies their general class; Figher, Ranged, Mage, Healer, and Creature are the ones that come to mind. A unit's class determines what levelup perks it gets access to and what item upgrades can affect it. A handful of regular buildings, and several special resource buildings, grant access to equipment upgrades, but only for the classes specified; some affect all units, others are specific to martial units or spellcasters. Units trained in a city with such an upgrade receive it automatically, otherwise it costs gold to upgrade an individual with the specified gear.
Sight Range and UpkeepMoving back between that and perks, we get Sight Range (most units have 2) and Upkeep. Both are fairly self-explanatory. Note that this unit requires both food and gold, as is typical for living, intelligent creatures.
ResistanceFinally, we have resistance. This is where things get interesting, because unlike HP, resistance is highly situational and altered by a wide range of effects. From my calculations, it seems as though each point (or rather, five points, since they don't come in lower increments than that) of resistance provides fairly steady benefits; that is, you should see about the same improvement going from 0 to 10 as you would going from 200 to 210, barring rounding errors. If you're familiar with damage mechanics of this sort, every 50 resistance appears to double your effective health against that damage type.
You might also notice that this particular fellow has 0 of every resistance against Life, against which he has 100. Most infantry starts out with an armor perk that improves their Melee and Ranged resistances; obviously, this archer isn't so lucky. His only resistance comes from being alive, which automatically grants 100 Life resist. Some of the more common traits are as follows:
Living units have 100 Life resist.
Undead units are immune to Death damage but suffer -25 Life resist.
Skeletal units possess 75 Missile resist.
Beast units suffer -25 Missile resist.
So, if you've actually read all this, you probably know almost all you need to in order to demand I build a silversmith on those pigs or move those wounded archers next to that enraged hill giant. If you just sort of skimmed it and maybe glanced at the pretty pictures... eh, good enough.