A seer is greater than a prophet… a seer is a revelator and a prophet also; and a gift which is greater can no man have, except he should possess the power of God, which no man can; yet a man may have great power given him from God.
But a seer can know of things which are past, and also of things which are to come, and by them shall all things be revealed, or, rather, shall secret things be made manifest, and hidden things shall come to light, and things which are not known shall be made known by them, and also things shall be made known by them which otherwise could not be known.
Thus God has provided a means that man, through faith, might work mighty miracles; therefore he becometh a great benefit to his fellow beings. (Mosiah 8:15-18)
A seer is greater than a prophet because he reveals the invisible. He has greater access to the mysteries of heaven. “Seers are able to see things that are not visible to the natural eye. Acting as revelators, they reveal what they see to the people.” (Dennis Largey, “Built upon the Rock,” Ensign, Jan. 1992, 51)
The best latter-day example of a seer is the choice seer, Joseph Smith. Joseph identified with Enoch as a seer. Both were shown marvelous things; both were called in their youth; both were given great power from God. In the early revelations of the Restoration, sometimes code names were used. The code name for Joseph was Enoch, suggesting that Joseph felt connected to this great prophet. (Richard L. Bushman, Joseph Smith: Rough Stone Rolling, [Random House, NY, 2005], 139)
For Joseph, his ability to see in the invisible was enhanced by a seerstone. Separate from the Urim and Thummim, the seerstone allowed Joseph to see things past, present, and future.
“Joseph received, apparently by chance, a stone found while he was digging a well for Willard Chase. While it looked like an ordinary stone, in his hands, through faith, he was able to learn to discern secret things. His mother said even before receiving the Urim and Thummim he ‘possessed certain keys, by which he could discern things invisible to the natural eye.’ [Lucy Smith, Biographical Sketches of Joseph Smith the Prophet, (New York, 1969) p. 92.] We do not know exactly how Joseph used this power, nor for what purpose. We do know that even before he received the plates he was able to tell the family about the customs and lives of the ancient inhabitants of America. Perhaps the Lord was revealing things through the seerstone.” (Richard L. Bushman, Joseph Smith Memorial Sermons, 9-10)
“[Martin Harris] said that the Prophet possessed a seerstone, by which he was enabled to translate as well as from the Urim and Thummim, and for convenience he [often] used the seerstone. Martin explained the translation as follows: By aid of the seerstone, sentences would appear and were read by the Prophet and written by Martin, and when finished he would say, ‘Written,’ and if correctly written, that sentence would disappear and another appear in its place, but if not written correctly it remained until corrected, so that the translation was just as it was engraven on the plates, precisely in the language then used.” (Testimonies of Book of Mormon Witnesses, 86-87.)