As scion of a mortal human and one of the last guardians, the human Mage Medivh comes under the influence of the dark Titan, Sargeras although it is revealed that he was possessed. Mad with lust for power, he offers the world of Azeroth to the bloodthirsty Orcs in exchange for the Titan's powers and unwavering loyalty of the Orcish Horde.
The Orcs begin a very swift invasion of Azeroth but are beaten back and Medivh himself is slain, putting an end to his quest for power and domination. However, fortunes take a turn for the worse when the Orcs assassinate King Llane, sack Stormwind Keep which is the capital of Azeroth and drive the humans from Azeroth north into the Lordaeron subcontinent.
Both Human and Orc harvest lumber, mine gold and build and repair buildings. Buildings enable creating more advanced units or researching technologies for improving weapons, shields or other abilities.
In combat, different units serve different purposes. Footmen (or faster and stronger Knights) are the basic fighting units; they are effective in finishing damaged units. Catapults throw missiles that deal heavy damage to nine squares at the same time; as they can not aim at an empty square, it is difficult for them to hit a moving unit.
Clerics and Conjurers are vulnerable spell casters that can heal damaged units, explore any segment of map, make a unit invisible, cast a rain of fire or summon Scorpions or a mighty Water elemental. In the game, their Orc counterparts, Necrolytes and Warlocks, have partly different spells, which they can summon Spiders turn dead units into Skeletons, make a unit invulnerable and summon a Daemon, the most powerful unit.
There are also several "neutral" units that appear in the dungeon maps, which include Brigands, Ogres, Fire elementals, Scorpion, spiders, slime and Skeletons. These have higher armor and deal less damage.
Featured in the single campaigns are a small number of Special characters that would evolve into heroes in further installments in the series, eventually leading to World of Warcraft. The only one of these characters to make later appearances is Sir Anduin Lothar who is featured in Warcraft II. The character of Medivh would also later appear in a prominent role in Warcraft III: The Reign of Chaos, and later in World of Warcraft: The Burning Crusade, where players must protect him from the Infinite Dragonflight in the Black Morass.
As a matter of fact, each unit is the same as its counterpart with the exception of some spells and only slight variations. For example, Orc Spearmen do slightly more damage than Human Archers while having a slightly reduced range.
In game, the summoned units are the only units which are exclusive and have no counterpart. While the Humans' scorpions and the Orcs' spiders are identical in hit points, attack power and speed, the Orcs can use a spell to summon skeletons from corpses, while the Humans' counterpart is a spell that heals units. In addition, the Demons and the Water Elementals are the most powerful units for each faction, are notably different. The Demons are slow melee attackers with more HP and damage output while the Water Elementals are ranged units. While subtle, the differences with these units provided something that would be kept in the series and give each race its flavor: the Orcs could wreak havoc using invulnerable Demons enchanted with Unholy Armor, while humans could simply pass by heavy defenses using Invisible Water Elementals.