Gacha derives from gachapon/gashapon, coin-operated vending machines containing an assortment of toy capsules. The capsules contain collectible items of various rarity. Each use, or "pull", of the machine gives you a random capsule so gacha enthusiasts may end up spending a lot of money trying to obtain rare capsules.
Many video games, particularly mobile games and MMOs, have implemented this model as a method of obtaining rare items and are thus called gacha games. Players often have a daily limit of pulls they can make, with the ability to purchase more pulls with real world money.
Use this tag for images related to gacha-style mobile games, and gashapon for physical vending machines or their capsules.