Very simply, rituals are the ways a given society applies human meaning to abstract symbols. We may think of ritual as something primitive, but in fact, it's all around us in our private and public lives. It might be something as complex and potentially dangerous as Alan's worship of the horses in Equus.
Public rituals are easier to define.
Superbowl Sunday is an American ritual. Churches, courtrooms and legislatures are important sites where ritual is used to give their proceedings added weight and significance. Rituals, when properly used, create tradition and traditions build community, tie generations together and connect people with a higher power, be it their government or their god.