Key differences exist between Sunnis and Shiite belief system with certain elements in Shiite practices can be traced back to Zoroastrianism, the pre-Islamic religion of Iran. The most visible of which is divination and communication with the dead which was widely practiced in ancient Persia. Shiites venerate Hussein because of his wedding with Shaharbano, the last descendant of Sassanid Persia of which The 12 Imams descended out of this union.

Quran

Shiites believe in the authenticity of Quran. However, one group known as Muta'awilah will always stir debate about Quran's authenticity when it was first written down. If certain practices or beliefs are in conflict with Quran then verses are taken out of context to justify that belief.

Sunnis unanimously agree that the Quran is authentic. Additionally, it is free from error and no verses were added, subtracted, or modified over time. Sunnis encourage that the book be read, interpreted and understood in Arabic.

Hadith

Shiites will only take Hadith referenced to the 12 Imams, Ali, and some of Ali's political allies. Attention to citation is minimal, and many books cannot be proved to be authentic due to weak citation.

Sunnis use Hadith as a second source for Shariah after the Quran. If a Hadith is agreeing with Quran, then it is followed. Conversely, If a Hadith was in disagreement with the Quran, then it is rejected. Citation was undertaken seriously by Bukhari and Muslim, if a citer is known to be a liar, or an unknown individual, the Hadith was rejected out of the collection.

Prophet's companions

Shiites consider the companions as infidels, save for a few. Ali has a special place in Shiism, some consider him an advisor, some a prophet, and some a God. Accusations of the companions as unjust Caliphs is full in Shiite historical writings. Shiites believe Ali should have succeeded the prophet after his death simply due to heredity.

Sunnis unanimously respect the companions and believe they were just. Caliphs were elected into power by public vote. Any disagreement that happened was due to struggling for keeping the truth and now it is over. Sunnis over the centuries advised Shiites that staying bitter over an event that spanned centuries is not acceptable.

Theology

Shiites believe in theism, but some practices falls into polytheism. For instance, prayers that should be directed to God are directed to Ali, Hussein, or Zainab, particularly when asking for help. Making an oath and sacrificing animals to other than God is practiced. Divination with the dead is practiced when visiting graves and there are many prayers and poetry that backs this practice. Additionally, the 12 Imams are believed to be infallible.

Sunnis believe in one God who is omnipotent, has no partner, no opposite, no equal, and no intermediary should exist between God and man when praying. Additionally, no one has the power to foresee the future but God, and all oaths, sacrifices, and asking for help should be directly done to and asked from God.

Predicting future events

Shiite believes that the 12 Imams have the power to see future events, and that Muhammad has no right to this power. Because of this view some Shiites will claim divinity for the 12 Imams.

Sunnis believe that this power is reserved to God and only prophets have foreseen the future through God's permission.

Seeing God

Shiite considers seeing God impossible in this life and the next.

Sunni consider Seeing God possible only in the after life, "Some faces, that Day, will beam, Looking to their Lord." (Quran, Resurrection, 23-24) Abu Sa'id Al-Khudri narrated, "O Allah's Apostle! Shall we see our Lord on the Day of Resurrection?" He said, "Do you have any difficulty in seeing the sun and the moon when the sky is clear?" We said, "No." He said, "So you will have no difficulty in seeing your Lord on that Day as you have no difficulty in seeing the sun and the moon (in a clear sky)."(Bukhari Volume 9, Book 93, Number 532s)

References

http://www.usc.edu/dept/MSA/fundamentals/hadithsunnah/bukhari/093.sbt.html#009.093.532s
http://www.rim.org/muslim/shiite.htm
http://en.wikipedia.org/wiki/Hadith_of_seeing_God_as_the_clear_as_the_sun
http://www.usc.edu/dept/MSA/quran/075.qmt.html
http://www.omania.net/avb/showthread.php?t=135826