I've heard different things regarding what's exactly in "it," but my Biology professor did mention the significance of the Skene's Gland in women, and how it is homologous to the male prostate gland. So lets say it's a sexy mix of urine and female prostate fluid.
From <a href ="http://en.wikipedia.org/wiki/Skene's_gland"> wikipedia </a> (NSFW, btw...but then again you wouldn't be looking at this at work, would you?)
"The fluid that emerges during female ejaculation has a composition similar to the fluid generated in males by the prostate gland,[6] containing biochemical markers of sexual function like Human Protein 1[7] and the enzyme PDE5. When examined with electron microscopy, both glands show similar secretory structures,[8] and both act similar in terms of prostate-specific antigen and prostate-specific acid phosphatase studies.[9][10][11][12] Because they are increasingly perceived as merely different versions of the same gland, some researchers are moving away from the name "Skene's gland" and referring to it instead as the "female prostate."
Bio prof's website with lots of fun information:
http://www.csupomona.edu/~gdbrum/bio301