Rauru is one of the sages featured in The Legend of Zelda: Ocarina of Time, and the human form of the owl Kaepora Gaebora. He is the first of the seven who you'll find out is a sage, which will occur automatically after Link meets him (whereas the others you'll meet before, though won't find out that they are sages until after the seven years pass).
Link first meets Rauru as the Owl Kaepora Gaebora. Throughout link's adventures as a child, Kaepora gives link advice. This is the only time Rauru is seen outside the sacred realm.
When Link entered the Sacred Realm, he was required to rest for seven years in order to grow older so that he could truly become a hero. Rauru kept him here for all this time, and explained to him what his obligation was - to first awake the rest of the sages and thwart Ganondorf. Following this he gave Link the Light Medallion (Rauru himself is known as the Sage of Light).
After sending Link off he awoke the other sages and requested that he find someone waiting for him at the Temple of Time. There he found Zelda. Doing so, however, revealed to Ganondorf the location of Zelda, and he quickly kidnapped her. With this, Link was required to save her after Rauru told him to do so. He and the five other sages opened the door to Ganon's Castle for Link, and in the end the seven sages (which included Zelda as well) sealed Ganon away. In the ending ceremony, six of the seven sages would overlook the celebration on Death Mountain, Rauru not included.