JOEL - I am not much of an authority of the Catholic faith but what I will try to do is take some general concepts and show how Catholicism differs from Mormonism (LDS).

GOD
Catholicism believes God to be the Creator of the universe, and that God's being is trinitarian; that the persons of Father, Son, and Holy Spirit exist simultaneously in one divine nature.
LDS doctrine is tritheistic. In other words, God the Father, Jesus Christ, and the Holy Spirit exist as three separate beings and are three separate Gods. The Son is subordinate to the Father, and the Holy Spirit is sent forth by the will of the Father through Jesus Christ, his son.

CHRIST
According to Catholic belief, Jesus was born of a virgin, and is the "Incarnate Son of God." As both God and man, he is the "Savior of the World."
For Latter-day Saints Christ was not, is not now, and never will be united in nature or substance with the Father. Jesus, in LDS belief, is the Only Begotten Son of the Father in the flesh. He entered mortality, subject to growth as well as being, and fulfilled the will of the Father as savior, and mediator.

ATONEMENT. In Catholic tradition Christ's Atonement provides access to saving grace. Christ's death-resurrection is the saving event and the cross, the symbol of salvation.
For Latter-day Saints the Atonement of Jesus Christ was a descending below all things in order to rise above all. He suffered "according to the flesh" because in no other way could he know the anguish of sin and sinfulness, exemplify redemptive love, and reconcile justice and mercy. The LDS do not use the cross as a symbol of their faith. To them the cross represents the death of Christ. LDS like to think of the resurrected, living Christ.
The Atonement reunites man with God both through sanctification and resurrection. All that Christ received from the Father may be received by man from the Father through Christ. Christ is not dead, but is a resurrected imortal being who lives today.

FORGIVENESS
The LDS believe that we can be forgiven for every sin we commit except for two. The taking of an innocent life(murder) and the unpardonable sin against the Holy Ghost.
The no forgiveness for murder, is binding only to someone who has already become a member of the LDS Church. Part of the repentance process that leads to forgiveness of a sin is the step called restoration or restitution; making right what was done wrong. Since man cannot restore a life, obtaining forgiveness in this life for murder, through the atoning sacrifice of Jesus, is impossible.
To commit the unpardonable sin against the Holy Ghost, a person must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against Him. As Mathew explains:

"Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come." (Mat. 12:31-32)

AUTHORITY
Authority from God which gives us permission to perform ordinances and preach the gospel in Christ's name is very important. In this regard there could only be two Christian religions who could possibly be qualified. The Catholics and the Mormon Church.
Catholics believe that Jesus bestowed his pastoral authority on Peter, who thus became the first leader of the church, and that this authority to teach and to sanctify has been passed on in unbroken succession in the institution of the Papacy.
The LDS believe that after the original twelve apostles died the priesthood authority was lost and Christ's church went into apostasy. Doctrines and teachings were lost or were changed by the beliefs or desires of man. The priesthood authority was restored to the earth when Peter, James and John appeared to Joseph Smith and laid their hands on his head and reconferred the priesthood.

SCRIPTURE
For Catholics, the Old and New Testament is the "inexhaustible source of Christian belief." There is no other scripture nor will there ever be.
For Latter-day Saints the canon remains open. Scripture is the record of prophetic utterance given under inspiration. There is no final revelation. Revelation is on-going. When a latter-day prophet speaks for God it is the same as scripture. LDS believe the Bible, Book of Mormon, Doctrine and Covenants, and Pearl of Great Price are all scripture.

EUCHARIST. For Catholic tradition, the Eucharist is a Sacrament in which the true body and blood of Jesus is physically present, that is, the actual saving reality of the Lord. The act of consecration is a true sacrifice in which, through transubstantiation, the elements of bread and wine become the body and blood of Christ.
Latter-day Saints understand the Sacrament as a remembrance of the body (bread) and blood (water) of Christ. Sanctification is from the Spirit and takes place in the recipients who bring a broken heart and contrite spirit to the prayer and the partaking. LDS partake of the Sacrament to remind them of the promises they made at baptism. LDS do not believe in transubstantiation.

OTHER DIFFERENCES
LDS give great honor to Mary, the mother of Jesus, but do not believe in the immaculate conception.
LDS believe in the premortal existence of the spirits of all mankind.
LDS believe the Fall of man as planned, voluntary, and essential to the growth of the soul.
LDS do not believe in the original sin and infant baptism.
LDS baptise by immersion by someone with priesthood authority.
LDS do not believe that there is only heaven and hell. There are several degrees of glory in the resurrection that people are saved in.
Catholics hold that marriage is a life-long contract that ends at death and do not permit divorce. LDS believe that the marriage can continue in heaven after death.