In Warcraft 2, humans and orcs have equivalant units. This means that although they have different names, and graphics, they both have an equal or similar role in what they do. In the more advanced sections of this strategy guide, I refrain from mentioning the equivalant units because it becomes unnecessary.
A unit consists of everything that you can build out of your buildings. They include peons/peasants, ships, ogres, mages, etc. Each unit you have costs you 1 food to support. That means the more units you have, the more farms you will need to support your army. Units are built using several different buildings. A barracks builds footman/grunts, archers/axethrowers, knights/ogres, and ballistas/catapults. A shipyard builds every type of ship. An inventor/alchemist builds flying machines/zeppelins and demolition squads/sappers. Mage Towers/Temples of the Damned build Mages and Death Knights respectively.
Units are built, by selecting the proper building, and then pressing the hotkey for that unit, or by left clicking the units icon. If you have the available resources and food requirements, then the unit will start production. Below are pictures of all of the human and orc units respectively.
Footman/Grunt - Barracks
With this is mind, you can see how killing certain buildings would hurt the enemy more than others. For example, killing the ogre mound or stables, stops all of the enemies barracks from producing knights and ogres. This becomes very important in games when you have an opportunity to take out some of the enemies buildings.