The highest priesthood office in the Church. Apostles serve for life and are “special witnesses” of Jesus Christ to the world. They fill the highest governing bodies of the Church. Three apostles form the First Presidency. Twelve more make up the Quorum of the Twelve Apostles.
See also Quorum of the Twelve Apostles
In the video below, a current apostle, Elder David A. Bednar, speaks about the role of an apostle in The Church of Jesus Christ of Latter-day Saints.