This is a list of in-game terms and what they mean.

Gold : In-game soft currency that can be used to purchase potions, enchant, reforge affixes, reroll values, and learn new skills.
Karat : In-game hard currency that can be used to purchase avatars, open chests, learn skills, and enchant items.
Stamina : Stamina is consumed to play the stages, and 5 Stamina are recharged every 30 minutes. If you don't want to wait, you can purchase Stamina with Karats or receive them as a gift from friends.
Sapphire : Sapphire is consumed to play Monster Wave. You can receive them by clearing stages or purchasing them in the Shop.
Brawler's Badge : A Brawler's Badge is needed to enter the Arena. They are dropped in Monster Wave or you can purchase them in the Shop.
Melee Badge: A Melee Badge is necessary in order to enter Melee Mode. 6 Melee Badges are given out everyday and you can receive more by buying them from the shop. 
Versus Ticket : 10 tickets are given out every day, and they can be shared by all characters linked to the account. A Versus Ticket is needed to participate in Versus mode.
Stage Mode : You enter stages using Stamina, and you will receive EXP and rewards for clearing stages.
Monster Wave : Sapphire is consumed to play Monster Wave. You fight wave after wave of monsters, and you will receive rewards depending on the total headcount of monsters killed.
Arena: Fight one-on-one with another user and receive rewards based on points earned.
Socket : Equipping jewels into sockets will increase your character's stats. You must have an empty socket to equip a jewel.
Quests : Missions you must fulfill.
Treasure : Treasure chests that produce random items. Can be opened with keys or Karats.
Avatars : A fashion statement as well as a way to increase your character's stats.
Craft : The function of creating new weapons, armor, or accessories out of crafting material.
Enchance : The function of leveling up the character's weapon, armor, or accessory.
Reforge : The function of changing an equipment's affix.
Reroll : The function of changing an equipment's affix value.
Inherit : The function of new equipment inheriting another equipment's enhance levels.
Meteorites : Equipping meteorites will increase your character's stats, like jewels do. You must have an empty spot to equip a meteorite.