Confirmation is a mature commitment to Christ by affirming one's faith, rooted in the baptismal covenant. Confirmation classes, called Rite Questions, are held for youth 13+ and adults who are ready to discover the meaning of Christian commitment in their lives and explore how that calls them to live.
Confirmation is a sacramental action of the Bishop, however there are two other similar rites for adults. If you were confirmed by another tradition (Eastern Orthodox, Roman Catholic, ELCA-Lutheran) then you would be Received. If you were confirmed in the Episcopal (or an Aglican Communion cousin) Church, but for whatever reason you desire to make a mature and focused committment, then you would Reaffirm with the Bishop. All of these happpen at the same time and have the same preparation, yet they are three separate 'outward signs of inner grace'.