The story follows the life of Tomoya Okazaki. In the first story arc, called School Life, he is a high school delinquent who meets many people in his last year at school, including five girls, and helps resolve their individual problems. The second story arc, called After Story, follows his life as an adult after graduating from school. The gameplay of Clannad follows a branching plot line which offers a number of alternate scenarios with dialogue choices and courses of interaction, focusing on the player character's interactions with various non-player characters.
Source: Wikipedia, "Clannad (visual novel)", available under the CC-BY-SA License.