The King is the most important unit in the game because the game ends when a king dies. If you lose you king, you lose the game; kill your enemy's king, and you win. It is a ranged unit with short attack distance. The King has low action points, a low attack rate and low health, but it has heavy armor and has the benefit of attacking twice in the same turn.
The swordsman is a melee unit. He has high health points and a medium armor, which makes him a strong combatant. Considering that the swordsman has a high rate of attack, armor and decent mobility, it can be a strong member of your army, while also presenting a challenge when faced as an enemy on the battlefield.
The archer is a ranged unit with a high range. In addition to their range of attack, archers have a high mobility, so they can cross large swaths of the battlefield in one movement if they have not previously shot that turn. However, because it's take a while to reload and aim the bow, archers expend a lot of action points on their shots. They are lights units who wear no armor and have low health points, so keep them back or defended, lest they end up on the business end of heavy hitting melee units.
The healer is a special unit. They have no attack and low health points. Additionally, healers have no armor, and only have a medium mobility. Their range of healing, too, is limited. So if you find yourself commanding healers, you need to keep them positioned in the optimal spot where they can be defended, but also reach your units when they are injured. Because healers can also add additional health to your units (even if their health is still at its max) they are often vital in an army’s success in the field of battle.