A riot is a violent outburst by a crowd. When the principal didn't declare a snow day, even with five inches on the ground already, students rioted, gathering outside and throwing snowballs.
The meaning of riot has stretched over the years. You would say "She is a riot" about a funny or outrageous person. A loud clothing combination might represent a riot of texture and color. When something runs riot, it has been allowed to get out of control.