basically there was a difference in opion about who should be caliph after the Prophet saws Abu Bakr ra was chosen but Shias say it should have been Ali ra, but some take it further and say that Ali ra was acctually the prophet and Jibril as made a mistake by giving the Quran to Muhammed saws, this is an impossibility as Jibrail can only follow the command of Allah as he is an angel, so they are saying Allah made a mistake, which cannot be??
also many shias worship Ali and his sons the Prophets gransons, Hasan and Hussain ra, they believe they are devine, Ali ra killed people who decided to pray to him, when he was alive, he was an amazing warrior and very wise man, so probably a man of great strength and carisma,however this doesnt make him worthy of worship, he had these people burnt after they refused to disist worshiping him, they said indeed you are God as only Allah punishes with fire, authu bilahi, they were very decieved by the shaitan
If you have studied the Life of the Prophet saws you will know that he was indeed a prophet of Allah, Ali was his cousin and never, declared he was a prophet or anything except an ordinary man,
I am a sunni, so i guess shias would say different, but just how christianity was twisted by paul, so was Islam by a man called Abdullah (sorry i cant remember his family line) he created a weakness in Islam by corrupting it and encourageing grave worship and shirk, which is shiaisim as it is today.
sunnis do not agree that shias are muslim as to worship someone other than Allah takes u outside the fold of islam, they worship Ali and his sons and sometimes his wife.
however i should add there is a small party of shias who simply believe Ali ra should have been the next caliph, though they DO accept the rulings of the caliphs inbetween Muhamed saws and Ali,
i sincerly hope inshaallah that made sense, any shias who are offended, maybe you shoud do your homework on the roots of shiaisim.