Quests are story lines that characters can follow in-game by fulfilling tasks assigned by certain NPCs. Rewards from completing quests vary widely but generally include XP, SP, Adena and items. The list of existing quests is viewable in-game in the Map window's World Info section, under the Quest tab. The list can be sorted by quest name, start level, and region.
To assist new players in exploring Lineage II, the early quests a new character encounters are explained in a detailed walk through in the Newbie Quest Guide.
Other activities similar to quests are Quest Zones. To learn more about this topics, please visit the dedicated page.