It typically is the slight bit of fluid that seeps out of the head of the penis during sexual interaction or arousal. This means that precum could actually come out of the penis if the guy sees a pretty girl on the street that he gets a hard-on for ;)
It looks like normal cum (clear-whitish) but it just seeps out from the head, like I said. It usually gathers at the top until there is enough to "roll" down the penis shaft or drip off.
Men cannot control when they precum or how much they precum. They are not aware of when they will precum or even if they have just released some precum. If the penis is already inserted into the vagina then you cannot visually tell if precum was released or not because it looks exactly like vaginal lubrication too.
Precum is basically the male body's way of preparing for complete ejaculation (the "spraying" you get when you give a handjob--the "spraying" that goes on during sex). Precum, typically, contains some live sperm. MANY women get pregnant each year from their partner releasing pre-ejaculate fluid in or onto their vagina.
If you get this on your hands and then touch your vaginal area you can still become pregnant--just like with "regular" cum. You can also contract STDs and STIs from this contact as well.
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