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.)