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1.
A series of danmaku shooting games made by one guy under the name ZUN. He is worshiped as a God on many image boards who view his work as some of the best shit ever.

Basically the game involves you flying around as a loli dodging bullets of many a color (blood red, royal blue, cum bubble white, etc.) while laying the beat down on other lolis who want to destroy you for their own personal gain. Usually.

Touhou is popular mainly because of the HUGE amount of fan works and shit other people put out for it, doujins, fan games, and even a trading card game. Tons of inside jokes revolve around the series and charicters and a lot of it is frowned upon outside the small circle of fans by people calling players and fans of the game "pedophiles" and "touhoutards" /a/ and /v/ boards on 4chan say they hate it but only because the majority of them suck nuts at it.

There is a Touhou Wiki and a deticated Touhou image board on Wakachan.
"I can't get past Touhou 8: Imperishable night stage 4 on easy guys." -typical /a/ poster.
"lulz, go back to tetris." -Typical /a/ troll. (Usually sucks at Touhou as well.)
by [DoA] Shunt October 16, 2007
 
2.
A modern Japanese cult centred around the god Zun. Touhou believers usually find enlightenment in hallucinations of "bullet hell games. Currently, Touhou believers are waiting for an astrological event known as the Thirteenth Game.
Don't you try to make me join Touhou, I'm a devout atheist.
by TheOriginalPokegami September 20, 2011