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.