Extracurricular names the things a student does at school or college that are not the lessons in the course — the club, the team, the play, the volunteer group. The course itself is the curriculum; extra- here means outside, so the word is outside the course. Though actually, schools still organise these things, and they often sit on a student's record next to the grades, so they are not extra in the sense of unimportant. It is mainly an adjective (extracurricular activities), and sometimes a noun for the activities themselves. Teachers, parents, and application forms use it; students themselves often just say clubs or after-school activities.