What are the central beliefs of Shia muslims? What are the central ideas the differentiate them from others?

Update: I work with a Muslim thanks for asking. I would like to know more about what he thinks. I already drive him crazy with questions and I dont what to make him feel picked on as I like the guy. I would hope this section would be helpful.
Update 2: Uniting muslims seems as productive an idea as uniting Republicans and democrats.
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