Let's get this out of the way right off the bat: Vaginal discharge is just part of life. Comedians joke about discharge, doctors say it's normal, and people with vaginas are used to seeing it in their underwear at the end of the day. But what exactly is the stuff? Is it water? Vagina juice? Sweat? All of the above? These are all good questions, and the answers can vary.
Normally, the glands on the walls of your cervix create a clear mucus that drains downward, according to Medline Plus. As the mucus exits your body, it mixes with bacteria and discarded cells. There are also glands on either side of the vaginal opening that secrete fluid that adds to your discharge, according to the Mayo Clinic. So, assuming you do not have a vaginal infection, your discharge is just a cocktail of cells, bacteria, and bodily "fluid."
When this mixture is exposed to the air, it might turn yellow or white, which is why it can look vaguely like cream cheese when it's dried in your underwear, according to Medline Plus. The amount, color, and consistency of this discharge varies from vagina to vagina, according to the Mayo Clinic — it can be white, clear, thick, watery, sticky, or smooth. It might have a scent, because of the bacteria, or it might be completely odorless. Its consistency can change throughout a menstrual cycle.
If you notice changes in your discharge that seem out of the ordinary, that could be a sign of a vaginal infection, such as a yeast infection or bacterial vaginosis. The former is caused by an overgrowth of yeast inside of your vagina, which can build up and end up in your discharge — but you'll probably know what you're dealing with based on the telltale itch. Your discharge will look noticeably different when you have a yeast infection, and almost like cottage cheese. (There's also a super-helpful visual of what this looks like in your vaginal canal, here.)
On the other hand, bacterial vaginosis, or BV, is caused by an overgrowth of certain types of bacteria. And if you have it, your discharge will have a distinct, fishy smell, and might look grayish or white because of the extra bacteria. While yeast infections can usually be treated with over-the-counter products, BV may require a prescription for antibiotics — so head to your gyno if you suspect it. And finally, your discharge is going to be different when you're on your period, because your vagina will shed uterine tissue, vaginal fluid, and bacteria in addition to blood.
Next time you see a smudge of discharge in your underwear, or maybe more than that or less, don't freak, because it's probably just mucous and cells, and is a sign that your vagina is functioning as it should be. Self-cleaning, which is really pretty handy.