Definition
Oral sex involves the stimulation of the genitalia of a person by another person using the mouth (including the lips, tongue and/or teeth). The oral stimulation of the vulva is called Cunnilingus (from the Latin cunnus, "vulva" and lingere, "to lick"), while oral sex performed on a male is called fellatio (from the Latin fellare, "to suck"). Another form of oral sex is the oral stimulation of a person's anus, called anilingus (from the Latin anus + -lingus, from lingere, "to lick"). [1] [2] [3]

A common slang term for giving oral sex to either a man or woman is giving head from the term head job (in contrast to hand job, manual stimulation). To give somebody a blow job is an act of fellatio and cunnilingus is sometimes referred to as muff diving, eating out or poon-job. Anilingus is coloquially referred to as rimming or rim job. [1] [3]

"69" is a sex position in which two people align themselves so that each person's mouth is near the other's genitals, simultaneously performing oral sex. The participants are thus mutually inverted like in the number 69, hence the name. This position can involve any combination of genders. [2]

Statistics
Oral sex is quite popular. One of the most comprehensive studies concerning this topic was done in the United States and comprises data of 13.395 participants. According to the National Health Statistics Report (2006-2008), 89% of women and 90% of men had at least once had oral sex in their lives. [4]

In Austria this form of sexuality is also appreciated as statistics show that 80% of all Austrians perform regular oral intercourse. [5]

Medicine
A new study carried out by the Kinsey Institute, Indiana, investigated the effects of regularly performed cunnilingus in 150 heterosexual and lesbian couples. Those who had more oral sex showed a higher sleeping quality and more intimacy. According to the results, the release of more oxytocin leads to the regulation of stress and the protection against cancer and heart diseases. [6]

Further Links:
Cunnilingus, sexplanation, 2015 (YouTube)
ORAL SEX!, anakednotin, 2012 (YouTube)