I get this question so many times when I play and also see this question in forum, so let me help to explain
Quest is basically what you need to level and help you the understand RO2 feature. If you read the story of the quest, you will also understand what is the background story of RO2. Some of the quest is daily quest (usually, you get it from the announcement board (first one, after Prontera map)) but mostly is one times only.
Q 1 What is the fastest way to leveling
A I believe the answer is quest, quest give you lot of combat experience and zeny. Also, sometimes: potion, food, buff item and equipment. Because of that, always clear the main quest in a map
Q 2 How to find the person/ monster/ item from monster / item from harvesting?
A left click on the quest window at right, it will display the monster position in map
for person we need to look/ report after the quest finish will appear as ???
for enemy we need to kill or item to search will appear as red dot
if there is no monster appear after you click, most likely the monster is not in the map (go to another map) or the monster is a dungeon monster
Q 3 can I abandon a quest?
A Yes, you can and later you can retake it by re-talk with the NPC (however, I don?t recommend it as the quest mostly don?t have a time limit, just let it stack in your quest window)
KHARA, let?s say this is a ?side quest? that give you combat and job exp, zeny and another name like ?poring hunter?, ?amateur craft?, etc
Q 4 Khara quest
A After you challenge it, you can make the Khara quest appear or gone from the Khara quest list by tick the quest in Khara quest pop up
after you make it appear, it have the same way with quest window to find the monster we need to kill.