Tawheed (Islamic monotheism) is the greatest foundation of Islamic belief. In fact, a person is considered as Muslim if he/she accepts that there is no god worthy of worship but Allah and that Muhammad is His slave and His messenger to humankind.
Almighty Allah Says in the Noble Quran (what means): "Allah- there is no deity except Him, the Ever- Living, the Sustainer of [all] existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and whatever is on the earth. Who is it that can intercede with Him except by His permission? He knows what is [presently] before them and what will be after them, and they encompass not a thing of His knowledge except for what He wills. His Kursi (literally, a footstool) extends over the heavens and the earth, and their preservation tires Him not. And He is the Most High, the Most Great." [Quran 2: 255]
In these marvelous verses, Allah describes Himself as The Only One- Most Powerful, Ever-Living Lord of us. Tawheed is the bedrock of Islam and it states that it is Allah Who has created the universe with whatever is in it and administers it. What we deduce from the operation of the universe and call ‘natural laws’ are, in fact, Allah’s regular ways of creating things and events and administering the universe.
According to Islam, all religions revealed to the prophets, may Allah exalt their mention, have the same essence and knowledge of Tawheed and unity of Allah, but, with the time, the message was misinterpreted, mixed with superstition, and degenerated into magical practices and some rituals. That was the same message with which Adam, may Allah exalt his mention, was sent down to earth, the same knowledge that Allah revealed to Nooh (Noah), Ibraheem (Abraham), Moosa (Moses) and 'Eesa (Jesus), may Allah exalt their mention, and the last Prophet sent to humanity, Muhammad .
Islam rejects characterizing Allah in any human form or depicting Him as favoring certain individuals or nations on the basis of wealth, power, or race. However, our human minds are often in search of understanding the concept of Allah in materialistic ways, although we are not capable to completely comprehend this concept. When Prophet Muhammad’s contemporaries asked him about Allah and Allah revealed a chapter for this question in the Quran known as soorah Al-Ikhlas (Chapter No. 112 of Quran) which is considered as the motto of Tawheed. In this chapter Allah the Almighty Says (what means): "Say (O Muhammad), “He is Allah, [who is] One. Allah, the Eternal Refuge. He neither begets nor was He begotten. Nor is there to Him any equivalent." [Quran 112:1-4]
The Creator must be a different nature from the things created, because if He is of the same nature as they are, He will be temporal and will, therefore, need a maker. If the maker is not temporal, He must be eternal. But if He is eternal, He cannot be caused. If nothing apart from Him causes Him to continue to exist, He must be Self-Sufficient and Self-Subsistent. If He does not depend upon anything for the continuance of His own existence, this existence can have no end. The Creator is therefore Eternal and Everlasting Allah Says (what means): "He is the First and the Last" [Quran 57:3].
The Creator does not create only in the sense of bringing things into being, in other words, He is not only a Starter, He also preserves everything, takes them out of the existence, and is the ultimate cause of whatever happens to them.
'Ali Ibn Abu Talib the fourth Muslim Caliph, is reported to have said: "He is being but not through the phenomenon of coming into being. He exists but not from non-existence. He is with everything but not physical nearness. He is different from everything but not by physical separation. He acts but without the accompaniment of movements and instruments. He is the One, only such that there is none with whom He keeps company or whom He misses in his absence."
In Islam, Allah is known by His Names and Attributions and the manifestation of these names in the universe.
Another aspect of Tawheed in Islam is that it implies the equity and unity of all people in their relation with Allah. Thus, people of different social strata were not created by separate deities with varying levels of power, since this would violate Tawheed by putting barriers between them. Instead, social dimension of Tawheed states that the same Allah created everyone, and so all people have the same fundamental essence. In fact, the noblest person in Allah’s Sight is the one who is most Allah-conscious.
Prophet Muhammad said: “Your Lord is One. You are from Adam and Adam was created from dust. An Arab is not superior to a non-Arab, nor a white person over a black person, except for his/her piety and righteousness.” [Ahmad] - islamweb.net