Tawhid is an Arabic word that is literally translated as “Doctrine of Oneness.” Tawhid is the manifestation that all Muslims make that there is only one God, and that God is unique, that there are no other Gods. Muslims call God “Allah,” which is simply the Arabic word for God. The word Allah is identical to the Hebrew word Jews use to refer to God, “Elloh.” Muslims believe that Allah is the same God that Jews and Christians pray to, although do not believe in the Trinity or in assigning any human characteristics of associated divinities to God.
The Qur'an forms the basis for Muslim's belief in the oneness of God. In the Qur'an it is said that there is a single, unique, independent, and transcendent truth, absolutely indivisible – the Creator of everything. Muslims believe that there is justice in the universe and that there exists a coherent and moral organization in the universe – that the universe is neither chaotic nor amoral. The Qur'an rejects the idea that God can be evil; arguing that all good comes from God and that evil is the result of His creation's free will. Muslims also believe that God is universal, that God does not apply to a specific group of people or only to the followers of a certain religion. Muslims believe that God is absolute and is the same God for everyone.
Tawhid is a fundamental belief of all Muslims. To become a Muslim, one of the first actions is to declare one's belief that there is only one God. The Qur'an mentions that the only unforgivable sin is to believe that there are other Gods. Everything in Islam, all of its teachings and moral framework part from the fundamental belief in a single, absolute God, from Tawhid. Islam is completely monotheistic, with absolutely no room to stray from Tawhid. The importance of monotheism in Islam is one aspect that sets it apart from most of the other major world religions.
For centuries, Muslim philosophers and intellectuals have expanded on the principle of Tawhid. While the concept is simple to understand: “There is only one God,” the implications and deeper meaning of God's oneness affect all branches of the Islamic faith and of the Muslim view of science, society, law, and science. An example of how the concept of Tawhid permeates every facet of Islam can be seen in Islamic art. Because God has absolutely no likeness, and because he transcends creation, Muslims are banned from painting, building, or sculpting an image of God. This ban later was extended to include depictions of the human form or of the prophet Muhammad, due to the possibility of it being viewed as a form of idolatry. This is why Islamic art is mainly centered on calligraphy and abstract figures.
-- Al Arabiya Digital