Fleet Girls Collection 艦娘
What's Kantai Collection?
Kantai Collection (aka Fleet Collection, or Fleet Girls) is a Japanese web browser game. (In Japanese, it's 艦隊これくしょん)
The game was launched in 2013. As of April 2015, the game has 3 million registered players.
Each World War 2 warships of Japan Navy are represented by a cute girl on a card. Each warship has very detailed info in the game. Players collects the ships/girls and battle each other.
The game is hugely popular in Japan (and have copy-cat games in China). The game also spun comics, TV series, toys (action figures), cosplay, etc.