(1) Discharge of semen out of excitement (ejaculation due to a wet dream etc.).
(2) Sexual Intercourse (penetration) (i.e. the male private part is inserted into the female private part). Ghusl becomes obligatory on both the male and the female even if there is no ejaculation.
(3) At the end of menstruation.
(4) At the end of nifaas.