What exactly is vaginal sex?

Vaginal sex, sometimes called penetrative sex, is when a person puts their penis inside someone's vagina. Vaginal sex is one of many types of sex. It is also the only type of sex that can lead to pregnancy. 

What is that embarrassing sound that sounds like a fart that sometimes happens when I'm having sex?

This is something called vaginal flatulence, sometimes called “queefing" in slang. It occurs when air gets into the body during arousal, penetration, or even exercise. As the air leaves the body, it can make an embarrassing sound.  However, this is a normal experience common to anyone with a vagina!

During sex, the vaginal canal lengthens and the uterus moves.  This can create extra space for air to collect. Often, people experience vaginal flatulence when the walls of the canal return to their unaroused state. However, it is also possible to experience flatulence from particular exercise positions, as in yoga.