Husayn was the second son of Ali and Fatima Zahra, and was beloved by his grandfather Muhammad. Many stories tell of a great resemblence between Husayn and Muhammad. There are numerous hadith that talk about Husayn and his elder brother Hasan and express the love that Muhammad had for them. In one of these hadith the Prophet said 'Hasan and Husayn are like my two sweet bazels in this world'.
After the death of his father in 661, the Caliphate was up for grabs and a conflict existed between those who thought Muawiyah should be the caliph, and those who felt that the son of Ali should take it. However, Husayns elder brother, Hasan, did not pursue the issue. When Hasan died though, Husayn took up his claim to the caliphate amid charges that the current caliph (Yazid I, the son of Mauwiyah) was corrupt.
Husayn thus led an insurrection and received support from the people of Iraq. However, when his forces met those of Yazid at Karbala, he was betrayed by those who had come to his aid. Husayn and most of his family were massacred at the battle. Husayn himself had his head cut off and delivered to Yazid. This event is known as Ashura and is a holy day for the Shia.
This episode of Husayns' life is of special importance to many Muslims for it's tragedy and for the bravery of Husayn in the face of an overwhelming opponent.