What was the most important difference between the Sunni and the Shia?

Sunnis and Shias primary differences is who they recognize in the line of succession for the prophet Muhammed.

Sunnis trace the succession to Abu Bakr, an advisor to Muhammed, while Shias trace the succession to Ali, a relative of Muhammed. They trace the leadership through the Caliphs, ending with the Ottoman Empire.

Shias recognize the descendants of Muhammed as their spiritual leaders and call them Imams.