"Rezu," as far as it's used to refer to a genre of Japanese media, is used to describe porn. "Yuri" is used mostly to refer to 2D works, the fictional love that you find in manga and anime.
In broader society, rezu is used to refer to, well, lesbians. Specifically referring to women who have sex with other women. It's also used as derogatory slang for lesbians. In general, whenever it comes up like this we usually handle it some other way (see: "are you one of them" from chapter one)
This is me trying to sum up a part of Japanese culture in like two sentences so I've missed a lot of nuance and context, but that's the general idea.
(Why didn't we just go with "les" or something? Uhhhhhhhhhhhhh...)