Vaginal farts, also called varts, queefs, or most formally, vaginal flatulences, refer to air being expressed from the vagina, which often makes a noise not unlike flatulence from the anus.
Vaginal farts can commonly occur during intercourse, or other kinds of sex play that involve penetration or even a lot of arousal and movement. Vaginal farts also occur during yoga (according to About.com's Guide to Yoga, there's even a separate yoga hold, called the mula bandha, that can be engaged to potentially reduce vaginal farts).
The expression of air out of the vagina is harmless, and because the air isn't from the anus or rectum, it should be mostly odorless. If an individual is experiencing vaginal flatulence and it is accompanied by a foul odor, it would be recommended that they speak with a doctor and have an exam, as it could be an indication of a problem.
Some people suggest that using more lubricant during vaginal intercourse, reducing the intensity of thrusting, and changing sex positions may all reduce the likelihood of vaginal farts. An equally useful approach can be to take it as a reminder that our bodies are never fully under our control, and giving and receiving pleasure doesn't always follow social convention.