The man who has been an apostle the longest is the president of the Church, and by inspiration he selects two other apostles as counselors.
These three function as the First Presidency, which is the highest governing body of the Church. (For information on the other 12 apostles, see Quorum of the Twelve Apostles, which is the second-highest governing body of the Church.)
Members of the First Presidency are special witnesses of Jesus Christ, called to teach and testify of Him throughout the world. They travel frequently, addressing and encouraging large congregations of members and interested nonmembers, as well as meeting with local leaders.
When they are not traveling, members of the First Presidency counsel together and with other general Church leaders on matters affecting the worldwide Church, such as missionary work, temple building, spiritual and temporal welfare and much more.