Originally, a series of bullet-hell shmups set in Gensokyo. A world separate from our own inhabited by every kind of supernatural creature mentioned in japanese folklore, and more.

Aside from the entries in the main series (of which there are many) Touhou consists of a litany of fan-made additional media, which is how it has ballooned into something that is really hard to explain.

The original solo creator, “ZUN” places almost no restrictions on others using the characters and settings.

Generally, you can think of it as bunch of fun characters that a bunch of people have used for their stories and games.