Alright, I'll try to do my best to explain about Touhou Project.
It's a game series developed by ZUN, and I think he may still be the only person programming them, but don't quote me on that.
It's style of play is top-down danmaku shooter, that is to say, bullet hell.
It has a pretty tight learning curve and feature some of the most gorgeous (and difficult) patterns for a player to master.
There are somewhere around 164+ characters and growing (this includes playable characters and bosses).
It was originally for Comodore PC, but was later developed for Windows with the release of the third game 5th game "Embodiment of Scarlet Devil".
The plot varies from game to game (if you want to call it a plot), but basically it's a shrine maiden and a witch trying to solve the problems of the fictional land Gensokyo, which is inhabited by Youkai and where spirits who are no longer believed in go to live their lives.
I hope this is enough information.
Sorry, it was late when I typed this. I wanted to say the fifth game, but had started to type third forgetting there were four in front of it. Uh, I've never played Raiden, but looking at screenshots I guess it's kind of like that.