In 610 CE, Allah ordained the final Prophet He would send to earth. Over five centuries had passed since the previous Prophet He sent, Jesus the son of Mary. This final Prophet was Muhammad ibn Abdillah (saas). Though Muhammad (saas) was born in the Arabian peninsula, he was sent to all of humanity. He was commissioned to deliver a message from Allah to Arabs and non-Arabs alike.
The message was the same one which Allah had sent down to the earliest humans. It contained a description of the way of life which Allah had established for people to follow while they lived here on earth. By agreeing to follow this way of life, people would essentially be submitting themselves to Allah. The name of this way of life was simply 'submission'. In Arabic, this is Islam (pronounced "Isslaam"). Today, Islam is found in all parts of the world, and its adherents are known as Muslims.
The message of Islam was conveyed in only two ways: in the form of a book, and in the living example of the Prophet. These two are the basis for a way of life which is acceptable to both mind and heart - it is neither illogical nor impersonal. It is in fact the only true way of life revealed from the Creator. The scope of this way of life is vast enough that it transcends the traditional notion of 'religion'. Islam includes submitting to Allah in the realm of politics, economics, law, etc.
Whoever wishes to understand Islam can only start at one place: the Oneness and Uniqueness of Allah. This is the essence of the creed of Islam, and no other creed on earth approaches its power and elegance. Scholars have written at length on the beliefs of Islam (e.g. Ibn Taimiyah and at-Tahawi), but the core belief is undisputed:
Sahih Bukhari - Volume 9, Book 93, Number 470: Narrated Mu'adh bin Jabal:The Prophet said, "O Mu'adh! Do you know what Allah's Right upon His slaves is?" I said, "Allah and His Apostle know best." The Prophet said, "To worship Him (Allah) Alone and to join none in worship with Him (Allah). Do you know what their right upon Him is?" I replied, "Allah and His Apostle know best." The Prophet said, "Not to punish them (if they do so)."