What was the argument about between the different muslims when Uthman (R.A) united them?

I know there was an argument between the Muslims in the subject of Quran at the time of Uthman (R.A). **What was the argument about**? Did the Muslims not agree that other recitations were correct ? And if so, Uthman (R.A) still left the marks and vowels out so that wouldnt solve the matter. I am so confused, please let me know.