Please also review the Basics of Islam chapter that addresses this subject too.
God is one; exclusive and only who does not require any help and has no associate or offspring. All other entities, physical, spiritual or any other shape or form seek and need His help and each and every entity depends upon Him. This concept of the Oneness of God, the Tauheed, is the basis of Islam.
Related to this, Allahu Akbar and La ilaha illallah are words frequently uttered by Muslims, and nowadays commonly heard on the media. A lot of time though, they seem mere utterances than the statement of belief or the deeper understanding they convey. Allahu Akbar sometime is loosely translated as “God is great,” implying (may God forgive us) that God is great like a mountain is great or a nation is great or a president is great. This is inappropriate and wrong understanding of what is actually said and implied. May be a closer translation is “God is the greatest;” even that does not really convey the full meaning. There is no entity or a concept that is superior to God in any perceivable manner. Everything we could fathom or think about is from God, He is the creator of everything, even our thoughts.
La ilaha illallah can be translated as “there is no other entity worthy of worship except Allah (the name for God in Arabic). These few words convey an acceptance of a fundamental understanding that there is no other person, not even a prophet, any entity, a concept, a philosophy, a physical or a conceptual reality supreme over Him. He is beyond any such comparison. Understanding this concept imply that He is the only one who we are commanded to worship and ask for anything our mind could think of.
God has clearly stated that he has no offspring or a relative like a parent. He is beyond these concepts. We are specially commanded to be careful to avoid worshiping or even giving an impression of worshiping anything other than God. We have to be extra careful when we study the lives of His prophets, all of them, or anybody who seems closer to God and avoid worshiping them or even including them in what is God’s dominion. We are told that this one particular act is unforgiveable.