Main quests are non-grind-able, NPC generated quests which all players are able to receive. Players can only attempt the quests when certain requirements are met, such as level requirement or the existence in guilds. They also serve as the backbone of the game, providing stories, conversations, or sometimes tips which could aid in your gaming experience.
Players are allowed to party up with others to complete a certain quest, be it dungeon-types or just an errand. However, players in the party which does not have the quest will not be able to receive quest-specific items. On the other hand, players with the same quest will receive quest-specific items at the same time, should any item be acquired.
Players will acquire certain artifacts or items after completing the quests, although there are some which does not give any rewards. Acquired rewards ranges from gold to equipment, potions to scrolls, etc. All quests also reward experience points for completion.