There are many monsters in the world of MapleStory. The most dangerous monsters live deep in dungeons.
Harmless monsters tend to stay by towns. Maple Island is one of the places that has very few monsters and are mostly very weak. Higher level monsters live in Victoria Island, but more dangerous monsters live closer to the center (Dungeon) of the island. Ossyria mainly has high level monsters as well as four major bosses.
After killing a monster, Mesos or monster leftovers (ETC. items) may be dropped sometimes. However, the monster may also drop something rare (infrequently of course), such as an equipment, which can be worth a lot. The mesos dropped by a monster is varied, and the range varies with the monster (and is usually, but not always, somewhat proportional to the level of the monster, so you will find that in SOME circumstances monsters of same level drops the same range of mesos), the maximum value usually being 1.5x the minimum value. Monster leftovers have many varying uses. Some are used for quests, exchange quests where large numbers can be turned in to an NPC for experience, or for forging materials, such as Firewood that is used to make Processed Wood. In versions with the Maker skill, 100 monster leftovers can be combined to form a "Monster Crystal", an alchemical reagent used to create items.
Monster Book
In KoreaMS, JapanMS, TaiwanMS, and ChinaMS, there is a system called the "Monster Book." Monster Book is, as the name suggests, a book which every player is given that catalogs monsters. When the Monster Book is added to the game, monsters will begin dropping "cards" that contain detailed information on them. Each individual monster's card can be collected up to five times. Upon picking up a monster's first card, the monster's slot in the book lights up and it's standing sprite becomes viewable. The second time a card is picked up, the monster's HP and MP values will be displayed, as well as some new animations of the monster, such as walking or knockback. The third time, the monster's history and level will be added. For the fourth card, a near complete drop list is revealed, with certain items (such as Skill books) omitted. The fifth card will reveal the monster's exact locations across the world, and grant a temporary 15% experience boost. This boost lasts for one hour, starting from the moment the card is picked up. All remaining animations for that monster will also be unlocked. Though cards will continue to drop after the final card is found, picking up more than five will have no effect. In addition to their permanent change to a player's monster book, each card looted will cast a "buff" on the player, granting them a helpful boost such as increased drop rates or speed/jump boosts when hunting in the region where that monster appears.
Cards in the book are sorted by relative level range. There are tabs on the side of the book which allow one to skip from level range to level range. The tabs on the book's side are color coded, and the colors match those in the background of the cards which appear in that section. A Chief Gray, for example, has a green background on its' card, and the tab under which it appears is also green. The only exception is cards from Boss monsters, as these appear in a different section, called "Special Cards". Certain event monsters also appear as special cards.
Only monsters found on Maple Island, Victoria Island, and Ossyria currently drop cards. In JapanMS and ChinaMS, cards drop from Zipangu monsters, and monsters from Shanghai Wai-Tan and Shaolin Temple drop Monster Book cards as well. All other World Tour areas, such as Floating Market, Singapore, and Ximending, do not have cards in any version. It is expected that when the version which created a town gets the Monster Book, the monsters in that version's World Tour area will begin dropping cards. Monsters in Party Quests do not drop cards, with the exception of bosses: King Slime, Alishar, Poison Golem, Lord Pirate, Papa Pixie, and Frankenroid. For players who are above the level range of the Party Quests, opportunities to "buy" these cards appear from time to time as part of events.
MapleSEA will be releasing New Leaf City and will be re-releasing Targa and Scarlion Boss. It will also improve some of the Fantasy Theme Park monster's HP/EXP ratio.
ChinaMS test server has released Elin Forest and Elin Forest Party Quest. They are also releasing Mu Lung Dojo, Mal Volence Party Quest and Family system.