Islam is a monotheistic religion based on the Quran. With approximately 1.2-1.3 billion adherents it is the world's second-largest religion. Islam is considered an Abrahamic religion along with Judaism and Christianity. Followers of Islam are known as Muslims. Muslims believe in only one God (Arabic: Allāh) who has revealed his word to humanity through many earlier prophets, and that Muhammad was the final prophet. Muslims believe that the core message of Islam, submission to God, has been the essential message in the teaching of all God's prophets.
 

The basic tenet of Islam is found in the shahādatān ("two testimonies"): lā ilāhā illā-llāhu; muhammadun-r-rasūlu-llāh — "There is no deity worthy of worship other than God (Allah) and Muhammad is a messenger of God (Allah)." A person who truly believes in the meaning of these words is a Muslim. However, for practical reasons one may need to recite the words in the presence of witnesses to be considered one by other members of their faith.
 

Muslims believe that God revealed his direct word for humanity to Muhammad (c. 570–632) and earlier prophets, including Adam, Noah, Abraham, Moses, and Jesus. Muslims believe that Muhammad is the Last, or the Seal, of the prophets. Muslims assert that the main written record of revelation to humanity is the Qur'an, which they believe to be flawless, immutable, and the final revelation of God to humanity.
 

The Five Pillars of Islam:
 

There are five basic beliefs shared by all Muslims, which are called the Five Pillars of Islam:
 

Shahādah: Testifying that there is none worthy of worship except God and that Muhammad is his servant and messenger.
 

Salah: Performing the five daily prayers.
 

Sawm: Fasting from dawn to dusk in the
month of Ramadan.
 

Zakāt: Giving Zakaah (alms giving).
 

Hajj: The Pilgrimage to Mecca during
the month of Dhul Hijjah, which is compulsory once in a lifetime for one who has the ability to do it.

Islamic Law:
 

The Sharia (Arabic for "well-trodden path") is Islamic law, as shown by traditional Islamic scholarship. The Quran is the foremost source of Islamic jurisprudence. The second source is the sunnah of Muhammad and the early Muslim community. The sunnah is not itself a text like the Quran, but is extracted by analysis of the hadith (Arabic for report), which contain narrations of Muhammad's sayings, deeds, and actions. Ijma (consensus of the community of Muslims) and qiyas (analogical reasoning) are the third and fourth sources of Sharia.
 

Islamic law covers all aspects of life, from the broad topics of governance and foreign relations all the way down to issues of daily living. Islamic laws that were covered expressly in the Quran were referred to as hudud laws and include specifically the five crimes of theft, highway robbery, intoxication, adultery and falsely accusing another of adultery, each of which has a prescribed "hadd" punishment that cannot be forgone or mitigated. The Quran also details laws of inheritance, marriage, restitution for injuries and murder, as well as rules for fasting, charity, and prayer. However, the prescriptions and prohibitions may be broad, so how they are applied in practice varies. Islamic scholars, the ulema, have elaborated systems of law on the basis of these broad rules, supplemented by the hadith reports of how Muhammad and his companions interpreted them.

There is no official authority who decides whether a person is accepted into, or dismissed from, the community of believers, known as the Ummah ("family" or "nation"). Islam is open to all, regardless of race, age, gender, or previous beliefs. It is enough to believe in the central beliefs of Islam. This is formally done by reciting the shahada; which should be made sincerely from the heart, the statement of belief of Islam, without which a person cannot be classed a Muslim. It is enough to believe and say that one is a Muslim, and behave in a manner befitting a Muslim to be accepted into the community of Islam.
 

More information on Islam at:
 

http://www.muslim-answers.org/Introducing-Islam/ii-toc.htm

http://www.islam-guide.com/