The Nasuverse is the setting that all the writings of TYPE-MOON's scenario director, Kinoko Nasu, share. It is made up of several "franchises" that all take place in the same world with the same rules, but interestingly enough, seldom cross over in an obvious manner. Also of note is that while these franchises are not conducive to separate continuities of each other, the franchises themselves are made up of several, since the primary works are visual novels with multiple and entirely mutually exclusive paths.
It's constructed mainly of several extremely popular franchises, like Tsukihime, Melty Blood, and Fate/Stay Night, Kara No Kyoukai being relatively less known, though DDD is by far the least recognized. Several of the main works are eroge (h-games, though it‘s more plot with porn than porn with plot), while many of the related works are not.
Having plenty of great characters and an endless supply of powerlevel debates, it's quite fun.