If you are a member of a religious group, but you are not an ordained minister or priest, then you are a member of the laity. Sometimes members of the laity will play a role in the church service, for example, doing one of the readings or running a youth group.
The noun laity is from the word lay, and lay person has a similar church meaning. The word lay itself is from the Greek word laikos meaning "of the people." The meaning of the word laity is sometimes expanded to refer to non-professionals workers, as opposed to professionals in a field. Volunteers are the laity of elementary education, and if you volunteer at a school, you can make a huge difference in a child's life.