It's a long story..
However, during prophet Muhammad (pbuh) time, the enemies of Islam were led by Abusufyan, the head of the Ummayds' tribe..
Shortly before the death of the prophet, the conquest of Mecca happened and the Ummayds declared themselves Muslims..
The Shiites believe that this declaration was not sincere, and just after the death of Muhammad (pbuh), the Ummayyads made a coalition with the hypocrites of the city of Medina..
Ultimately the Ummayds ruled first Syria and fought imam Ali, and then ruled the whole Muslim empire and massacred the prophet family and their Shiites, where the word Shiite in Arabic means follower.
The Sunnis consider the Ummayds as legitimate Caliphs..
The Shiites believe that the prophet appointment the 12 imams from his progeny. They believe that the imams are similar to the prophets in Judaism.
They also believe that the imams were appointed leaders like king David in Judaism..