Dungeons are a primary form of end-game content in World of Warcraft and have been in the game since launch. They are instances that can be completed with a 5-player party - a healer, a tank, and 3 DPS.
In classic World of Warcraft, there was only Normal mode for dungeons. However, dungeons were still very challenging at this point.
In The Burning Crusade expansion, Heroic mode was added for dungeons. This allowed players to complete more challenging dungeons for better rewards.
In the Mists of Pandaria expansion, Challenge mode was added for dungeons. In Challenge mode, players must complete the dungeon within a specified amount of time. Successfully completing the dungeon in the amount of time given would result in the player receiving cosmetic gear.
In the Warlords of Draenor expansion, Mythic mode was added for dungeons. This allowed players to complete very challenging dungeons with a premade group to receive some of the best endgame gear outside of raiding.
In the Legion expansion, Challenge mode was removed and Mythic+ mode was added. In order to participate in a Mythic+ dungeon, a player must complete a dungeon on Mythic difficulty. The final boss of the dungeon has a chance to drop a Mythic Keystone. The keystone will start at Mythic Level 2 and only be usable for a specific dungeon. The player must then enter the dungeon on Mythic difficulty and place the keystone in the Font of Power near the entrance. Similarly to Challenge mode, the group only has a certain amount of time to complete the dungeon. If the group succeeds, they will receive a chest with gear at the end of the dungeon, and the keystone will be upgraded either one, two, or three Mythic levels depending on how fast the dungeon was completed. If the group fails to complete the dungeon in the amount of time allowed, the keystone will become depleted, but they will still receive a gear chest at the end of the dungeon. Players can attempt to run the dungeon again with a depleted keystone. Should they succeed this time, the keystone will be recharged and upgraded, and they will also receive a loot chest again. Additionally, players are able to open a chest in their order hall the following Tuesday to receive a piece of gear equivalent in item level to gear from the highest level Mythic+ dungeon they completed that week, as well as a new keystone, regardless of whether they completed the dungeon in the time limit. Modifiers to make the dungeon even more difficult are applied at Mythic Levels 4, 7, and 10. The lowest Mythic+ level is Mythic Level 2, and the highest is Mythic Level 15.