World of Warcraft is easily the most popular subscription-based Massively Multiplayer Online Role Playing Game in the world today. It's based on Blizzard Entertainment's popular Warcraft Real Time Strategy game series and is set seven years after Warcraft III.
World of Warcraft is based in the world of Azeroth, on three main continents: Kalimdor, the Eastern Kingdoms, and Northrend; as well as Outland, whence the Orcs originate. There are two factions: the Alliance and the Horde (though neither is automatically good or evil -- and in trope terms, both are The Alliance), twelve races - split evenly per faction, ten classes, ten primary professions, and four secondary professions. The game is casual-friendly to a point, being designed so that those who prefer to play solo can still achieve maximum level. It should be noted that, like most MMORPGs, about half of WoW's content can only be accessed after you reached maximum level, and cooperative play is required to earn the best gear and other rewards. Guilds are a major part of the game's community, with the majority of players belonging to one. Benefits of guild membership are both social and material -- most endgame raid content is done by guilds and they can share a pool of resources much greater than that available to the solo player. The Cataclysm expansion added more benefits to being in a guild, ranging from faster movement speed, extra gold from killing monsters, and in some cases, mounts - should your guild attain a high enough rank. WoW's classes follow the standard RPG archetypes. The three core roles are tanking (taking damage so others don't have to), healing (patching others up), and DPS (optimizing the Damage you deal to the enemy Per Second, sometimes pronounced "deeps" for fun). These roles are further refined by the talents each character chooses to specialize in. There are "pure" classes that can only fill a DPS role, and "hybrid" classes which may either specialize in one role or fill multiple roles at the cost of utility.
- Death Knight (added in Wrath of the Lich King) -- A tank or melee DPS class that utilizes the power of death and disease, and may summon undead minions to aid it. Formerly servants of the Lich King, they broke free of his control and now seek to Pay Evil Unto Evil. Unlike other classes, new Death Knights start at level 55, but before a Death Knight can be created on an account, another character on that account of any class must be leveled to 55.
- Demon Hunter (added in Legion) — Based on the demon hunters of Warcraft lore, they utilize demonic magics and transform into demon-like forms to fight in melee. They are the second "hero class" after Death Knights. Unlike other classes, they only have two specializations: Vengeance for tanking, and Havoc for damage. For lore reasons, they are the most restricted class in terms of race; they are only available to Night Elves and Blood Elves.
- Druid -- Servants of nature, Druids are the most versatile class, relying on talents and shapeshifting to specialize in melee DPS (cat), ranged DPS (Moonkin), tank (bear), or healer (tree). Additional forms allow them to breathe underwater, increase foot traveling speed, and fly without a special mount.
- Hunter -- A ranged DPS class that tames beasts to use as pets in combat. The Hunter can specialize in improving his pet or his ranged combat skills, and is a master of traps and controlling monsters. Notably the only class that deals primarily physical ranged damage.
- Mage -- A ranged DPS class that excels at dishing out magical damage. Talent choices focus on specific types of magic: Arcane, Fire, and Frost. Mages are also versatile in managing large numbers of enemies and controlling monsters.
- Monk -- (added in Pandaria). They can tank as a Brewmaster, deal damage as a Windwalker, and mix up damaging attacks with magic to heal as a Mistweaver. They use energy and a new resource called Chi to mix up attacks. They also know karate.
- Paladin -- A hybrid melee class that can specialize as tank, melee DPS, or healer. Paladins wield their faith in the Light as sword, shield, and bandage, incorporating holy magic into their martial abilities as well as their spellcasting.
- Priest -- A hybrid caster, Priests channel their faith into spellcasting. The archetypal and most versatile of healers, they're the only ones to have two skilltrees dedicated to healing and protecting. Priests may also specialize as ranged DPS, destroying their enemies with shadow magic and psychic assaults.
- Rogue -- A melee DPS class that attacks from stealth for incredible damage, then vanishes into the shadows when the advantage is lost. Rogues may specialize in the use of poisons, toe-to-toe combat, or trickery and deceit.
- Shaman -- A caster/melee hybrid that draws on the power of elemental and ancestral spirits. Shaman may specialize in melee DPS, ranged DPS, or healing, and utilize mystical totems to buff allies and attack their foes.
- Warlock -- A ranged DPS class that employs demons and attacks with shadow and fire magic. Warlocks may specialize in summoning demonic minions to cause damage and bolster their party, withering their targets with curses, or battering them with direct power. Warlocks also have the ability to utilize fragments of their past victims' souls to a gambit of group-supportive ends.
- Warrior -- A melee hybrid, warriors are the pinnacle of martial prowess. Besides tanking, they may specialize in two forms of melee combat; either skillfully wielding a single heavy weapon, or relentlessly striking with two. Warriors utilize specialized combat stances to access and enhance various sets of offensive or defense abilities in the heat of battle.
Players can play against the environment, completing quests and conquering dungeons; against each other, in dedicated Battlegrounds and Arenas (and even in the world at large if playing on a server which allows such activity); or a mixture of any of these, with talents and gear to support either choice. Servers are designated Player Versus Player, Player Versus Environment, and Role Playing. The main difference is that on PvP servers, you are automatically flagged for PvP in all zones above level 20, and in PvE servers, you must manually flag yourself for PvP. RP servers can also be PvP or PvE, but have more rules in regards to character names, and for killing players much lower level than you.
The original, or "classic/vanilla" game, featured a level cap of 60 and was played in the two continents of Kalimdor and the Eastern Kingdoms. New content was added regularly up until the first expansion; the final released dungeon was Naxxramas.
The first expansion, The Burning Crusade, was released in January 2007, raised the level cap to 70, and allowed travel to Outland. Flying mounts and druid flying form were first introduced in this expansion, as well as an expanded world to fly within. It also allowed the creation of Blood Elf characters on the Horde and Draenei on the Alliance. This gave the Horde access to Paladins and the Alliance access to Shaman, a restriction which had previously caused game-balance issues. The final released dungeon of The Burning Crusade was Sunwell Plateau.
The second expansion, Wrath of the Lich King, was released in November 2008, raised the level cap to 80, and allowed travel to a new area: Northrend. It added the Death Knight "hero" class, available only to players who already had a level 55 or higher character. Death Knights started at level 55 in an exclusive zone with a full set of equipment, but no profession skills except First Aid. The final dungeon of Wrath of the Lich King is canonically Icecrown Citadel; although additional content was added to tide players over until Cataclysm, it is officially part of the latter. The last added dungeon is The Ruby Sanctum.
The third expansion, Cataclysm, was released in December 2010. It returns players to a redesigned Kalimdor and Eastern Kingdoms (devastated after the titular Cataclysm), raised the level cap to 85, and revisits a great deal of old and unfinished content while continuing the story lines of Azshara, Deathwing, and the Guardians of Tirisfal. The expansion also saw upsets in the leadership of several player races and a resurgence in the Alliance/Horde conflict storyline throughout Azeroth. New possible races are the bestial Worgen of Gilneas for the Alliance, and the cunning Goblins of Kezan for the Horde. A patch released shortly before the expansion also added a large number of new race/class combinations, explained in the lore as races attempting new things in order to survive the Cataclysm.
The fourth expansion, Mists of Pandaria, was released in September 2012. It raised the level cap to 90 and opened up the continent of Pandaria for exploration. It featured the Pandaren as a playable race for both the Alliance and Horde, a new class in the form of the Monk, and more focus on PvP content. Additional features included pet battles, PvE scenarios, and Challenge Mode dungeons. The final dungeon was Orgrimmar itself, with Garrosh Hellscream channeling the powers of an Old God in an attempt to take over the world for his New Horde.
The fifth expansion, released in November 2014, is Warlords of Draenor, raising the level cap to 100 and sending players to an Alternate Timeline version of Draenor in which Garrosh Hellscream travels through time to stop the Old Horde from drinking the blood of Mannoroth. His Iron Horde, equipped with modern technology, now threatens to storm through the Dark Portal to conquer present-day Azeroth. While exploring Draenor, players build and customize their own Garrisons and interact with legendary characters such as Grommash Hellscream and Ner'zhul. The expansion also features redesigned models for all the classic character races.
The sixth expansion, Legion, was released in August 2016, raising the level cap to 110, and sending players to the Broken Isles in the Great Sea to do battle with the Burning Legion, which is attempting its third invasion of Azeroth. Features of this expansion include the new Demon Hunter hero class, a revamped honor system, class-based orders headed by players, and a specialization-based Artifact-quality weapon, which has its own plotline and scales along with the player over the course of the expansion.
Well-known for its depth of interaction, WoW has also spawned a collectible card game, a pen-and-paper RPG, comic books, and many other merchandising peripherals. The game is currently the largest MMO in the world by a huge margin, currently having around 10 million players and at one point nearly 60% of the total market share, and a film is in the works. 
- For reference, the largest US MMORPG before this was EverQuest at half a million during its height; the largest worldwide was Lineage at 2-3 million -- although Asian MMORPG numbers such as Lineage can be considered inflated, as they have a different pricing structure in Asian markets that revolve around cybercafes. Note that World of Warcraft is played in the same cybercafes on the same terms.