The following makes the Ghusl obligatory:
1) The ejaculation of semen, regardless of whether one is awake or sleep:
Umm Salamah narrated that Umm Sulaym said, “O Messenger of Allah! Allah does not shy away from the truth. Does the woman have to make ghusl if she has a wet dream?” He replied, “Yes, if sees the liquid.”
Recorded in Bukhari, Muslim and Tirmizi
A’ishah said, “The Messenger of Allah was asked about a man who found wetness but did not recall experiencing a wet dream. The Prophet stated, ‘He must make Ghusl’. Then he was asked about a man who experienced such a dream but found no fluids and he said, ‘Ghusl is not obligatory upon him.’”
Recorded in Tirmizi and Abu Dawood. Graded Sahih by albani.
2) Sexual intercourse even if one does not ejaculate:
Abu Hurayrah reported that the Prophet said, “If one places himself between her four parts (that is thighs of his wife) and has intercourse with her, then ghusl is obligatory whether or not he ejaculated.”
Recorded in Muslim
3) A non-Muslim embraces Islam:
Qays ibn Asm stated that when he embraced Islam, the Prophet ordered him to make Ghusl with water and lotus leaves.
Recorded in Nasa’i, Tirmizi and Abu Dawood. Sahih by albani.
4) When the menses or bleeding after giving birth has come to an end:
A’ishah narrated that the Prophet said to Fatimah bint Abi Hubaysh, “When he menses come upon you, leave the prayers. And when the (menses) leave you, then make ghusl.”
Recorded in Bukhari, Muslim, Abu Dawood, Tirmizi and Nasa’i. Other than Bukhari they have “And wash the blood from you.”
5) Death (Washing the dead body):
Narrated Um ‘Atiyya al-Ansariya: Allah’s Apostle came to us when his daughter died and said, “Wash her thrice or five times or more, if you see it necessary, with water and Sidr and then apply camphor or some camphor at the end; and when you finish, notify me.” So when we finished it, we informed him and he gave us his waist-sheet and told us to shroud the dead body in it.
Recorded by Bukhari