There are a variety of chests that can be encountered in the game.
Chests are found on certain floors within dungeons, usually after beating a tougher-than-normal monster or group of monsters. These chests award XP, Gold and a handful of other items such as gems, armor, or weapons.
Chests can also be earned by completing quests. Quests have a set reward which can be seen on the quest information page.
Finally, there are 3 different kinds of chests that can be encountered within. Two chests require MP to open and can reward gold and/or XP.
The more elaborate chests require a key to open and are typically found in more populated areas. These chests always contain gold and can also contain gems and/or equipment, much like a chest you would open in a dungeon.
Mana chests give a small amount of XP and can on rare occasion also contain gems, weapons, or armor.
Ornate chests, which require keys to open, can only be opened by one person at a time. After you open a chest, there will be a count down of 1 min before the chest will replenish. If another player raids the chest before you, you will be notified and your key will not be used.