The Concept of GOD in Islam is Tawheed (Monotheism in Islam).
The very basic understanding of the concept of THE GOD (Allah in Arabic) in Islam is based on a key belief in the One God, called “Tawheed” or monotheism in English. However, the concept of Allah (the God) in Islam is uniquely understood and best described in the Arabic language as “at-Tawheed al-Islam” or “Unique Oneness of God.” It comes from the Arabic root word for “wahed” (one) and it carries three strong implications which originate from the same word
“Tawheed al-Rububiyah” provides us with the clear unmistakable understanding that there is only One True Lord who is the Creator and Sustainer of the entire universe. He Alone, is the Owner of all that exists; He is the Only Planner and Organizer; and He Alone is the Giver of Security. That whatever exists is His creation and what ever comes about, whether perceived by us as either “good” or “bad” is actually having it’s origin from Him Alone. That He Alone is “Allah” (The One God) and as such, He has no partners or associates or helpers of any kind. Allah say in the Quran: “All praise and thanks be to Allah, the Lord of the world” (Al-Fatihah 2:2). Allah also say : Bless is He in whose hand is the dominion, and He is able to do all things (Al-Mulk 67:1)
“Tawheed al-Uluhiyah” means that any and all worship must only be directed to Allah, directly without any intercessors or partners alongside of Him. Any worship, asking for help from the unseen, swearing, offering sacrifice, giving charity, fasting, pilgrimage and other acts of worship must all be offered only to Allah. Anything other than this would be considered as making “partners” alongside of Allah (The God) and would not be acceptable.
If anyone would believe for instance, that something within the creation of the Almighty might benefit them, or help them, or aid or protect them, then this would never be accepted as true worship which is indicated by “Tawheed al-Uluhiyah”.This might well be summed up in the phrase: “The Worship of the Creator and not of His Creations.”
Allah say: O humanity. Worship your Lord who created you and those before you so that you may become pious. He who has made the earth a resting place for you and the sky as canopy, and sendtdown water from the sky and brought forth therewith fruits as provision for you. The do not set rivals with Allah while you know (Al-Baqara 2:21-22)
This tawheed al-Uluhiyah is the reason behind the creation of jinny and mankind as Allah say: and I (Allah) created not the jinns and humans except they should worship Me (alone) (Ad-Dhariyat 51:56)
Tawheed al-Asma was-Sifat
“Tawheed al-Asma was-Sifat” is actually one of the most important, yet most understood aspects of “Lordship” in Islam. And this is the area where many go astray in their thinking and beliefs. Herein we will be able to see how the various divisions amongst the “monotheists” have occurred. Let us bring the understanding from these three aspects:
a) Allah must not be named or qualified by anything other than what Allah Himself and His Messenger, peace be upon him, have named or qualified Him. No attribute or quality can be offered in explaining or describing the Lord of the Universe unless He or His Prophet Muhammad, peace be upon him, have already offered this description.
One example might be that Allah has described Himself as “Having the Power to Do Whatever He Wills to Do” (Allahu ‘ala kulli shay in Qadir), but we would not offer more than this, for example by saying: “Allah can do anything.”
The difference here is that, while Allah is saying that He is fully capable of doing what every He Wills to Do, He would not “Will to Do” anything which would make Him no longer be Allah. For instance He could not lie; He does not cheat; He can not die and He can not be something in His creation or subject to the laws of His creation.
He could not be for example, a human walking on the earth eating, sleeping and worrying about what will happen next. This would naturally make Him no longer God, The Creator, Sustainer, Organizer, Planner and Protector of All that Exists. Pardon this poor use of English in the phrase, but: “He could never – NOT BE GOD.”
No one may attribute Him with any name and attribute other than He already explains in the Quran and His messenger in the hadith. He say that: And (all) the most beautiful names belong to Allah, so call on Him by them and leave the company of those who belie or deny His names. They will be requited for what they used to do (al-A’raf 7:180)
b) No one or no thing in the Creation of Allah may be named or qualified with His names or qualifications. As an example Allah has Named Himself “Al Kareem” (The Generous). Likewise, He has described Himself as “As-Sabur” (The Patient), And while any of us could be considered as generous or patient, we could never be considered as THE ABSOLUTE IN GENEROSITY or THE ABSOLUTE IN PATIENCE, because we could not be absolute in any characteristic. It is only Allah (The God) who can be absolutely perfect in any and all aspects. He say that : There is nothing like unto Him, and He is the all Hearer, the all Seer (As-Shura 42:11)
c) As an example, Allah Has said in His Last Revelation the Holy Quran, that after He created the Heavens and earth in six “yawm” (even periods of time or ‘days’), He rose above and positioned Himself over His Throne.
Yet the scholars of Islam would immediately agree that it is not acceptable to speculate on “how” He looks like or in “what” manner He accomplished this.
(edited from: http://islamtomorrow.com/tawheed.htm) by adding some verses