Third Orders are comprised of lay men and women who are formally associated with a religious order such as the Benedictines, Augustinians, Carmelites, Dominicans, Franciscans, Mercedarians, Norbertines, Servites, Marists, and Trinitarians and who may or may not live in community (Third Order Regular versus Third Order Secular).
Canon Law defines Third Orders as "associations whose members, while living a secular life, strive after Christian perfection by observing a papally approved rule, under the direction and spirit of a religious order" (Canon 702). Although members, sometimes call tertiaries, are not bound by public vows, they are fully members of a religious family.
Entry into a Third Order requires spiritual discernment and a proscribed period of formation, followed by a formal profession that includes promises or vows.
The list that follows links to articles on Secular Third Orders.