The concept of monotheism (known as tawheed in Arabic) is the single most important concept in Islam. Monotheism points to the first of the Ten Commandments, and in Islam everything is built upon the oneness of God. Islam calls humanity away from the worship of any part of creation to the worship of The One and Only True God. No act of worship or devotion has any meaning or value if the concept of monotheism is in any way compromised.
Due to its importance, the concept of monotheism (divine unity and singularity) must be properly and fully understood. For ease of discussion, monotheism can be looked at from the following three perspectives:
The Oneness of God in His Lordship (Omnipotence)
The Devotion of All Worship to The One God Alone
The Uniqueness and Oneness of God in His Names and Attributes
This breakdown is by no means the only way to approach the subject that God is one and unique, but it allows the topic to be easily analyzed and discussed. (Monotheism is the key to understanding Islam, and revisiting this concept is recommended.)