What is touhou?
Touhou is a series of bullet-hell games created by a guy known as Zun.
You control an anime girl and shoot at fairies and other anime girls while dodging the stuff that they shoot at you. The games are known for their extreme difficulty as well as good music and characters.
Gameplay
In the typical Touhou game, you have to progress through 6 levels that increase in difficulty as they go. The enemies will fire bullets at you which create patterns which you have to avoid as you fire at the enemies. As you progress, the patterns become harder. When you get hit, you lose a life, and when you lose all of your lives you have to start over. There is also an extremely difficult extra stage that you can play after you have beaten the game. There are 4 difficulties, easy, normal, hard and lunatic. Higher difficulties have more complex patterns and greater bullet densities.
Community
Touhou due to the music and the characters, Touhou has gotten a large fan base in japan. There is a large amount of fan made content for Touhou, games, art, music, doujins and even a fan made anime. In terms of popularity, touhou surpasses most anime, even some of the top like Attack on Titan. It also has a fanbase in the West, though do to the type of game and the fact that up until recently, none of the games had been translated to English or released over here, the western fanbase is pretty small.