Different people define "sex" in different ways. We will go into various definitions in further modules. Vaginal sex refers to when sexual contact occurs between individuals involving penetration, especially the insertion of a man's erect penis into a woman's vagina, typically culminating in orgasm and the ejaculation of semen. This form of sexual intercourse can cause pregnancy.
However you define it, being sexual with another person takes a lot of responsibility. It is important that sex occurs consensually, and that all consequences (including pregnancy and sexually transmitted infections) are understood.