Well after the Prophet had dided Shias say that he had,before he had died, appointed Ali as his successor. During his funeral processions there had been a meeting on who should be the caliph, despite the Prophet saying that his successor is Ali (who was at the funeral procession) So what happen was that they decided Abu bakar was to be the leader despite Ali and his family members saying he was the rightfull successor. This continued 3 times, until the 4th time people of the city elected Ali to be the nxt leader. This was because the lost coonfidence in other caliphs as they used to kill a lot of people and were not really the nicest of people. Sunnis however believe that the 3 caliphs after Muhammad were rightfully chosen.
I have vast knowledge on Islam. If you have any other questions please feel free to ask, i would LOVE to answer any other questions.
After Prophet Muhammad pbuh the Caliphas were... Abu Bakhr, Umar, Uthman &Ali .. Shias formed their 'party of Ali' and they curse the other 3 Caliphas in their friday khutbahs cos they believe it should've been Ali all along but the three Calipha's 'purposely didn't choose him' .. Shias confuse me.