If Scripture is one of the lungs of the Body of the Church, then Tradition is the other! As Catholics, we believe that Scripture and Tradition are so intertwined that together, these give us our identity. Tradition (with a Capital "T") are those things which we believe come down to us through Jesus, the Apostles, and their successors, the Bishops in union with each other, and are unchanging.
St. Peter is the first Pope, and the other 11 Apostles are the first Bishops. All bishops in the Catholic Church today are direct successors of the Twelve Apostles. When all the Bishops of the Church together with the Pope as head make a proclaimation, it is called the "Ordinary and Universal Magisterium" of the Church, and is held as Church Law.
Since the Council of Jerusalem in 50 A.D., to the Council of Trent in 1545 A.D., to the Second Vatican Council in 1962, the Church has a rich history of Tradition of the Bishops of the Church and the Pope coming together to govern the Church and the Faithful entrusted to their care!