First, a quick look at the basic dungeon sets, which were often numbered among the Tier gear in earlier days.
Dungeon Set 1 An eight-piece set from Old World, level 58-60 dungeons. You'll sometimes hear this set called "Tier 0" by players.
Dungeon Set 2 Patch 1.10 added quests to upgrade Dungeon 1 items to form this eight-piece set. This set is sometimes called "Tier 0.5" by players.
Dungeon Set 3 These are five-piece sets that drop in The Burning Crusade instances. While any class can equip any item, most are designed with specific classes in mind. The "class" sets drop in non-heroic instances, while other pieces can drop in heroic instances. This set is sometimes called "Tier 3.5."
Several other faction-based armor sets, such as the Zandalar Tribe set from Zul'Gurub, used to be referred to in the Tier progression. These fell out of the progression once the Tier terminology started becoming more standardized and widely used.
Tier 1 This eight-piece set drops in Molten Core.
Tier 2 An eight-piece set, Tier 2 drops in Molten Core, Onyxia's Lair and Blackwing Lair.
Tier 3 This set actually offers nine pieces for each class. They are quest rewards from items that drop in old, 40-man Naxxramas. Since Patch 3.0.2, Tier 3 gear is no longer available.
Tier 4 With Tier 4, raiding gear entered The Burning Crusade era. This is a five-piece set, sometimes with multiple spec sets for a class, obtained by turning in tokens that drop in Karazhan, Gruul's Lair and Magtheridon's Lair.
Tier 5 Again offering different sets for different talent trees, these five-piece sets are purchased with tokens that drop in Serpentshrine Cavern and The Eye.
Tier 6 Tier 6 offers eight-piece sets for multiple specs for purchase with tokens from Hyjal Summit, Black Temple and Sunwell Plateau.
Tier 7 Welcome to the modern era, baby! The five-piece Heroes' sets come from tokens dropped in 10-man ("regular") Wrath raids. Gloves and chest tokens may also be purchased with Emblems of Heroism. Archavon the Stone Watcher in Wintergrasp sometimes drops a random Tier 7 piece.
Tier 7.5 The five-piece Valorous sets are purchased with tokens dropped in 25-man ("heroic") Wrath raids. Leg and shoulders tokens may also be purchased with Emblems of Valor.
Tier 8 and Tier 8.5 Coming soon from Ulduar!
Finally, we leave you with a special tip for casuals, slowbies and completists: Do you drool over armor sets? Check out wowwiki's Armor Sets page for history, drop details and screenshots of matched sets that you can wear as early as your teens. After all, nothing says HAWT like a full set of Defias Leather.