A world boss is a high level boss designed to be fought by a large number of players. They have a fixed spawn schedule and location. The current world bosses in the game are Kzarka, Karanda, Kutum, Nouver, Watcher Offin Tett and the prior known as Event/Raid Bosses: Muraka, Quint and Vell. The majority (except the last three) drop yellow grade weapons (primary, secondary and awakening).
The system notifies all players in a channel the moment the world boss/es spawn with a roar and a message in the upper part of the screen. After 15 minutes if the boss/es aren't dead they will despawn.
Karanda spawns in the highest peak of Karanda Ridge in northeastern Calpheon. This boss drops yellow grade awakening weapons, which require level 56+ to use, as well as completing a quest to unlock the skill tree.
Nouver spawns to the southeast of the Sand Grain Bazaar, inside the desert. Nouver drops yellow grade off-hand weapons.
Watcher Offin Tett
Watcher Offin Tett spawns in the Holo Forest. It drops yellow grade primary weapons.
Quint, Muraka and Vell
While Quint, Muraka and Vell have fixed spawn times (before the other world bosses had them), only spawn once per week as an event, were referred to as Raid Bosses and Event Bosses in early statements and do not share the same type of unique loot the others have (yellow grade weapons) they're now called World Bosses and are included in the World Boss timetable.
Quint spawns in Quint Hill.
Vell spawns in Vell's domain in the ocean.