Catholic Social teaching is old. It is as old as the Church. Though the name and modern concept are quite new (that is in comparison to the age of the Church because modern Catholic social teaching is over a 120 years old) the roots are found in the earliest doctrines of the Catholic Church and of course in the teachings of Jesus. Modern Catholic social teaching finds its start with the Encyclical of Pope Leo XIII, Rerum Novarum (On the Condition of the Working Classes), put out in 1891. If you would like to learn more about the origin of modern catholic social teaching I would suggest this article by Fr. Paul Pearson: http://www.catholicculture.org/culture/library/view.cfm?recnum=7732 .
Catholic social teaching today is based upon seven major themes that are based on our understanding of human dignity and life rooted in the Catholic Church’s commitment to the poor and building relationships of life and justice. The seven themes are the life and dignity of the human person, the call to family, community, and participation, the rights and responsibilities of the human person, options for the poor and vulnerable, the dignity of work and the rights of workers, common solidarity of humanity, and care for Gods creation. For more on these I suggest looking at the USCCB’s page titled Care for God’s Creation (http://www.usccb.org/beliefs-and-teachings/what-we-believe/catholic-social-teaching/seven-themes-of-catholic-social-teaching.cfm)