it's basically a larger quantity of the same mucus-type fluid that is constantly in the vagina. there are glands in the vulva (cool name for the female genitals) that produce it.it varies from having no taste to being slightly tangy when everthing's clean and healthy, and can be anywhere from clear to white-ish and has varying degrees of thickness.
it's acidic (pH is about 5), compare that to semen which has pH 7. the pH prevents the growth of too much yeast/bacteria which as we all know leads to that tuna-factory smell.
another thing that a lot of guys don't know is that not using a condom with their woman can actually *contribute* to the change in smell, because it changes the pH of things in there.
all in all, as long as it's clear or white and doesn't smell *bad* things are pretty ok. if it stinks, or if it's some other color (aside from that lovely shade of red once a month) start worrying.
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kuru
'dancing is the vertical expression of horizontal desire.'
-robert frost