In the religion of Islam, sects are not really "denominations," if that word is understood to mean various valid approaches to the same religion. The division between Muslim sects - mainly Sunni and Shia - is deeply rooted in Islamic history. Debates revolve around key historical figures and important core beliefs, all of which have significant implications for how Muslims are to live in the world today.
Members of one Islamic group do not usually recognize members of other groups as fellow Muslims, and open conflict between sects is not uncommon.
Yet, numerically, it could be said that Islam is less divided than many other faiths because the vast majority of the world's Muslims are Sunnis.