13 Now Ammon said unto him: I can assuredly tell thee, O king, of a man that can translate the records; for he has wherewith that he can look, and translate all records that are of ancient date; and it is a gift from God. And the things are called interpreters, and no man can look in them except he be commanded, lest he should look for that he ought not and he should perish. And whosoever is commanded to look in them, the same is called seer.
14 And behold, the king of the people who are in the land of Zarahemla is the man that is commanded to do these things, and who has this high gift from God.
15 And the king said that a seer is greater than a prophet.
16 And Ammon said that 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.
17 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.From this we learn: A) Properly understood, a seer will have an interpreter of some kind to look in and use in this role, although it is uncertain whether or not this is a must or not B) Due to the fact that a seer can look into the past as well as into the future, this role is greater than the role of a prophet. We know that Joseph Smith used this role in translating the Bible, but we do not know conclusively whether or not presidents of the church after him have used this role. Brigham Young at one point showed a seer stone to some saints in the tabernacle, but it is unclear if he simply had the stone or used it.
As for a revelator, as the name implies this means that the president of the church (like any other member) can receive revelation that he is entitled to. The difference between the president of the church and other members is that he can receive revelation for the entire church, while others can only receive it for themselves and for their own stewardship.
Now that the roles of prophet, seer, and revelator have been established, the second question dealing with leading astray and erring can be addressed. It is a common belief that since a prophet cannot lead the Church astray, he cannot err in any great manner. For this reason, some have said the priesthood ban, while a painful thing to deal with, was justified rather than wrong.
This idea of not leading the church astray was talked about by President Wilford Woodruff after the adoption of the Manifesto in 1890. He said
"The Lord will never permit me or any other man who stands as President of this Church to lead you astray. It is not in the programme. It is not in the mind of God. If I were to attempt that, the Lord would remove me out of my place, and so He will any other man who attempts to lead the children of men astray from the oracles of God and from their duty."So, here we learn something we often do not talk about. Notice that President Woodruff said that the president of the church would have to attempt to lead to lead the Church astray in the second and third lines of the quote. In other words, the Lord will not allow a president to willingly deceive the church, but it does not follow that a president of the church cannot err in a major way. For instance, it clearly was wrong for Brigham Young to ban blacks from being ordained, but Brigham Young sincerely believed that they shouldn't be ordained; he was not attempting to deceive anyone. So while he erred and led the Church off the beaten path, he did not willingly lie about something and then present it as truth.
The bottom line is that God has given us prophets, bishops, other leaders, scriptures, prayer, and other methods to help us in our mortal journey. All of these things are useful and will help us when used properly. Where we often get off path is when we attempt to make these gifts into God and assume that because he is infallible the gifts are as well. Within a sphere, these gifts are beautiful and helpful, but like any tool they must be used in the proper way to be useful, otherwise they can be destructive.