There are big enough differences that sunnis see shiie as heretics and idolators and shiee see sunnis as lost and in need of guidance (but really heretics).

The conflict comes down to succession after muhammad. Sunnis believe abu bakir was the rightful heir, with Omar, uthman and ali following him as righteous kaliphs.

Shiee believe ali and his children (the exact children the believe to be heirs are different among sects) were the heirs and abu bakir, omar and uthman were all evil usurpers.

Now this seemingly small difference has created many bigger differences in laws and the way they conduct religious ceremonies.

It essentially all boils down to who was the rightful successor of prophet Muhammad. A classic tale of my guy is better than your guy.