The main spark that started the war was control of the Shatt-al-Arab waterway between Iraq and Iran, which is Iraq's only means of reaching the Persian Gulf. The underlying, larger cause, was that Iraq and Iran are traditional enemies, with Iraq being ruled by Sunni Muslims, and Iran by Shia Muslims. Iraq under Saddam Hussein mistreated its majority Shia population, and Iran gave money and assistance to potential Shia rebels. No love lost at the time between those two.
The war did not widen beyond those two countries, although Saudi Arabia, Egypt and Syria gave military and financial aid to Iraq at one point, as did the United States, who liked having Iran kept militarily busy defending its own borders. The war was almost an old style World War I bloodbath. Hundreds of thousands on each side were killed over eight years, and nothing was resolved. Each side economically and militarily tired of the fighting, and signed a cease fire.
This war was a war between Iran and Iraq. Iraq was supported by the United States during much of this war. The war began in 1980 and ended in 1988. I do not think you can say that either side won this war -- the outcome was just a whole lot of people getting killed.
The war started when Saddam Hussein, then leader of Iraq, decided to invade Iran in September of 1980. He did this because Iran had just had an Islamic revolution the previous year. Hussein was afraid that the new Iranian government would stir up the Shi'a Muslims in Iraq. They were the majority of the country but had little power under Hussein's rule. So Hussein attacked Iran in hopes of preventing Iran from causing a rebellion among the Shi'a.