While campaign games can have many different objectives, the sole objective in melee games is to destroy all the opposition buildings. In default melee matches, players can pick their own heroes, and losing one will not end the game. To make the game proceed more quickly, by default the map is covered in fog of war instead of the Black Mask. Warcraft III, like Blizzard's previous title StarCraft, allows for single and multiplayer replays to be recorded and viewed, allowing a game to be played at slower and faster speeds and viewed from the perspective of all players. Like all previous Blizzard titles since Diablo, Warcraft III uses the Battle.net multiplayer network. Players can create free accounts in regional "gateways," which helps reduce lag; these are Azeroth (U.S. East), Lordaeron (U.S. West), Northrend (Europe), and Kalimdor (Asia). Unlike previous Battle.net-enabled games, Warcraft III introduced anonymous matchmaking, automatically pairing players for games based on their skill level and game type preferences, preventing players from cheating and inflating their records artificially. If players want to play with a friend in ranked matches, Warcraft III offers "Arranged Team Games", where a team joins a lobby and Battle.net will search for another team; as with anonymous matchmaking, the enemy team is not known beforehand. Players can also host custom games, using maps either created in the Warcraft III World Editor, or the default multiplayer scenarios. The game also offers Friends Lists and Channels for chatting, where players can create custom channels or join Blizzard-approved ones. Warcraft III also allows players to band together to form "clans", which can participate in tournaments or offer a recreational aspect to Warcraft III. Global scores and standings in matchmaking games are kept on a "ladder". These rankings can be checked online without the need of the game.

Due to the latest patch, version 1.24, many third-party programs have been rendered unusable. Several third-party programs that reveal the entire map, commonly known as maphacks, have been released for the update. It also disabled collided maps, which would make modified custom maps appear to be the same as the original. Another effect of the patch, which is not included in the release notes, is that custom maps with large filenames will not appear in the game. The limit is believed to be 20 characters, but this has not yet been tested.

This patch also rendered many custom maps unplayable due to custom map scripts. Even some versions of the famous Defense of the Ancients were no longer functioning.

Warcraft III takes place in the fictional world of Azeroth. Several years before the events of the games, a demon army known as the Burning Legion intent on Azeroth's destruction corrupted a race called the Orcs, and sent them through a portal to attack Azeroth. After many years of fighting, the Orcs were defeated by a coalition of humans, dwarves and elves known as the Alliance; the surviving combatants were herded into internment camps, where they seemed to lose their lust for battle. With no common enemy, a period of peace followed, but the Alliance began to fracture. The events of Warcraft III occur after a timeskip from Warcraft II. This period was originally intended to have been documented in Warcraft Adventures, but that game was canceled in mid-development.

The game's plot is told entirely through cinematics and cutscenes, with additional information found in the Warcraft III manual. The campaign itself is divided into five sections, with the first acting as a tutorial, and the others telling the story from the point of view of the humans of Lordaeron, the Undead Scourge, the Orcs, and the Night Elves.

The game opens with the Orc leader Thrall waking from a nightmare warning him of the return of the Burning Legion. After a brief encounter with a man called "the Prophet", and fearing that his dream was more of a vision than a nightmare, he leads his forces in an exodus from Lordaeron to the forgotten lands of Kalimdor.

Meanwhile, the Paladin, Arthas defends the village of Strahnbrad from demon-controlled Orcs.. He enchants his warhammer via slaying Searinox, the black drake and one of King Deathwing's finest warlords. He then joins Archmage Jaina Proudmoore, who aids him in investigating a rapidly-spreading plague which kills and turns human victims into the undead. Arthas kills the plague's originator, Kel'Thuzad, and then purges the infected city of Stratholme. Jaina parts ways with him, unwilling to commit genocide, or even watch him do so. The Prophet, after previously trying to convince other human leaders to flee west, begs Jaina to go to Kalimdor as well. Arthas pursues the dreadlord Mal'Ganis, who was the leader behind Kel'Thuzad, to the icy continent of Northrend, where he helps his old friend Muradin Bronzebeard find a powerful sword called Frostmourne. Meanwhile, Arthas begins to lose his sanity, burning his ships to prevent retreat even when given an order to leave. Fortunately, Arthas and Muradin find Frostmourne and soon Muradin learns that the sword is cursed; Arthas disregards the warning and offers his soul to gain the sword. By doing so, Muradin was struck down by a shard of ice when Frostmourne is released, and is presumably killed. Arthas kills Mal'Ganis and abandons his men in the frozen north as his soul is stolen by the blade, which was later revealed to be forged by the Lich King. Some time later Arthas returns to Lordaeron and kills his father, King Terenas.

Now a Death Knight, Arthas meets with the leader of the dreadlords, Tichondrius, who assigns him a series of "tests". Arthas first exhumes the remains of Kel'Thuzad, contains it in a magic urn of the ashes of his father, which was protected by Uther. Arthas killed him too, then set off to Quel'thalas, kingdom of the high elves.He then later attacks the gates and destroys their capital of Silvermoon. He kills Sylvanas Windrunner, the Ranger General of Silvermoon (only to resurrect her as a banshee), corrupts their sacred Sunwell and revives Kel'Thuzad as a Lich. The Lich informs him of the Burning Legion; a vast demonic army who are coming to consume the world. Kel'Thuzad's true master is the Lich King, who was created to aid the Legion with his Undead Scourge, but in truth he wishes for the Legion to be destroyed. Arthas and Kel'Thuzad open a dimensional portal and summon the demon Archimonde and the Burning Legion, who begins his purging of Lordaeron with the destruction of Dalaran. Arthas and Kel'Thuzad were cast aside by Archimonde, and Kel'Thuzad reveals to Arthas the Lich King has already foreseen it and is planning to overthrow the Burning Legion.

Thrall arrives on Kalimdor, meeting Cairne Bloodhoof and the tauren, and clashes with a human expedition on the way to find an Oracle. Meanwhile, the Warsong clan are left behind in Ashenvale to build a permanent settlement, but anger the Night Elves and their demigod Cenarius by cutting down the forests for resources. To defeat them, the Warsong leader Grom Hellscream drinks from a corrupted fountain of health contaminated with the blood of the Legion's pit lord commander Mannoroth, successfully killing Cenarius, but binding his clan to the Legion's control. Thrall manages to reach the Oracle, in fact the Prophet, who tells him of Grom's doings. Following the Prophet's directions, Thrall and Jaina join forces to purge both Grom and the world of demonic influence. They succeed in capturing Grom and healing him of Mannoroth's corruption. Thrall and Grom begin to hunt Mannoroth and Grom kills him, dying in the process, but in doing so freeing the orcs from the demonic control of Mannoroth at last.