I came to the horrible realization that people who do not know what the Touhou Project is might end up here. So I suppose a quick rundown is necessary.
The Touhou Project, also known as the Shrine Maiden Project is a series of games that are generally vertical-scrolling danmaku games. Danmaku (lit. fire curtain) or "bullet hell" is a subtype of shoot 'em up games that focuses on dodging massive quantities of projectiles. Games of this genre are notoriously difficult, and Touhou games are no exception. The series also includes "danmaku photography games" and fighting games along the mainline games. At this point there are 19 mainline Touhou games and 13 spin-offs. This is quite an impressive number, and it's even more impressive that these games have been made by a single person frequently called by his pseudonym ZUN.
The Touhou games are set mainly in Gensokyo, which is an "otherworld" separated from the rest of the world by a magical barrier. This barrier enables magic to flourish in Gensokyo as it nullifies "common sense" that dictates that fantastical things shouldn't happen. The land of Gensokyo is populated by all kinds of mythical and magical entities, in particular youkai, fairies and deities. They have come to Gensokyo to escape extinction, as in the world of Touhou the existence of such entities depends on humans believing in them.
Most of the Touhou games have Reimu Hakurei, a shrine maiden and a "youkai exterminator" and Marisa Kirisame, a magician, as their main characters. Together they and occasionally other characters resolve various "incidents" which threaten the balance of Gensokyo. These incidents are resolved via a ritualized form of magical combat called "danmaku". In the lore danmaku is judged by how beatiful it is. In games, this is reflected by the aesthetic quality of the enemy attack patterns. This system was created to give humans a fighting chance against more magicaly powerful entities. At the end of every game, the incident is resolved and the involved parties generally come to amicable terms.
There are also a number of official print works associated with Touhou. They generaly represent darker takes on the setting of the series, and explore perspectives not seen in the games. The worldbuilding and narratives of the games and print works are notoriously contradictory by design and fans have interpreted the setting in a wide variety of ways.
The lore and worldbuilding of Touhou draws widely from Japanese mythology and religion, but also from more Western-influenced paranormal, esoteric and conspiracy lore ideas. Certain darker elements of the settings are essentially a critique of the present day condition and corruption of political and religious institutions.
ZUN has been very encouraging of fan works, and as the series is part of the Japanese culture of self-published media (doujin), it has enjoyed a very involved, creative and passionate fanbase for a very long time. Massive amounts of Touhou fan works have been produced, from art to entire fangames. On some level, this site too is a Touhou fan work I suppose, albeit a very unusual one...
If you would like to learn more, check out the Touhou Wiki
