The underlying causes of the revolution are described in another answer. What literally started the Iranian Revolution were popular protests against the Shah of Iran. The security forces would kill some of the protestors. In Shi'a Islam, there are traditional mourning days of 3, 7, and 40 days after deaths. The 40th day is the biggest day of mourning. Essentially, most of these mourning days would become new protests, more people would be killed, which increased the number of mourning days, which increased the number of protests, etc. Eventually, the security forces refused to fire upon their fellow Iranians and the Shah fled the country.
The Iranian or Islamic Revolution began in 1979.
1979
The commoners
The Iranian revolution was about the commoners being freed from the Shah.
It established an Islamic Repbulic.
Ayatolloah Khomeini was an Iranian religious leader in the 1979 Iranian Revolution. He managed to become the supreme ruler of Iran after the revolution.
Ruholla Khomeini
Leading the Iranian Revolution in 1979. He was also a religious leader.
Mohammad Reza Pahlavi was a ruler of Iran who was overthrown by the Iranian Revolution in 1979.
The Shah had a strong Western and Modern perspective. He was replaced by the Ayatollahs who had a strong Islamist perspective. Therefore the Iranian Revolution was seen as a symbol anti-Westernization and anti-Modernity.
Before 1979, Iran was very friendly and welcoming to the US. Iran was a very close ally of the US. After the revolution of 1979, the Iranian government declared the US a satanic nation.
Thousands died in the Iranian Revolution. The US hostages were all eventually released alive, but several American servicemen were killed when two aircraft collided during a failed rescue attempt.