Who is Allah?

Meaning of the Word Allah:
Allah is the Arabic word for "God." It is also considered by Muslims to be one of the names of God. Therefore, whenever Muslims mention Allah, they are simply referring to the one true God. In fact, even non-Muslim Arabs such as Christian Arabs use the word Allah to refer to God.
The term Allah has no plural or gender. This shows its uniqueness when compared with the word god which can be made plural, gods, or compared, goddess. To a Muslim, Allah is the Almighty, Creator and Sustainer of the universe, Who is similar to nothing and nothing is comparable to Him. Islam rejects likening Allah to any of his creation.

Why We Usually Write "(swt)" After Mentioning Allah:
Most Muslims append "(swt)" after mentioning Allah. This is an acronym for the Arabic words: "Subhanahu Wa Ta'ala," which roughly translated means "The most glorified, the most high." It is a way for Muslims to glorify Allah when mentioning His name.

Is Allah the Same God Worshipped by Christians and Jews?
Muslims believe that there is only one true God. While various nations and people claim to have various God's with various names, attributes and abilities, Muslims believe that there is in fact only one God.
Therefore, while Muslims do not agree with others about some of the attributes of God, we believe that there is only one God. For example, Muslims do not agree with the Christian belief of trinity, and in fact this is considered blasphemy as Islam teaches that there is only one God, not three.

Allah (swt) as Described in the Holy Qur'an:
The only source of information we have as humans about Allah is through divine knowledge sent to mankind through messengers. The holy Qur'an, which is the word of God revealed to humanity through God's messenger, gives us some information about Allah:
Allah (swt) said: "Say: He is Allah, the One and Only; Allah, the Eternal, Absolute; He begetteth not, nor is He begotten; And there is none like unto Him." (Holy Qur'an, Chapter 112, verses 1-4)
Allah (swt) said: "Allah is the Creator of all things, and He is the Guardian and Disposer of all affairs. To Him belong the keys of the heavens and the earth: and those who reject the Signs of Allah, it is they who will be in loss. Say: 'Is it someone other than Allah that ye order me to worship, O ye ignorant ones?'" (Holy Qur'an, Chapter 39, verses 62-64)
Allah (swt) said: "Behold! they fold up their hearts, that they may lie hid from Him! But even when they cover themselves with their garments, He knoweth what they conceal, and what they reveal: for He knoweth well the (inmost secrets) of the hearts. There is no moving creature on earth but its sustenance dependeth on Allah: He knoweth the time and place of its definite abode and its temporary deposit: all is in a clear Record." (Holy Qur'an, Chapter 11, verses 5-6)
Allah (swt) said: "No son did Allah beget, nor is there any god along with Him: (if there were many gods), behold, each god would have taken away what he had created, and some would have Lorded it over others! Glory to Allah! (He is free) from the (sort of) things they attribute to Him! He knows what is hidden and what is open; too high is He for the partners they attribute to Him!" (Holy Qur'an, Chapter 23, verses 91-92)
Allah (swt) said: "He is God; there is no god but He, He is the Knower of the unseen and the visible; He is the All-Merciful, the All-Compassionate. He is God, there is no God but He. He is the King, the All-Holy, the All-Peace, the Guardian of Faith, the All-Preserver, the All-Mighty, the All-Compeller, the All-Sublime. Glory be to God, above that they associate! He is God the Creator, the Maker, the Shaper. To Him belong the Names Most Beautiful. All that is in the heavens and the earth magnifies Him; He is the All-Mighty, the All-Wise." (Holy Qur'an, Chapter 59, verses 22-24)
Allah (swt) said: "There is no god but He, the Living, the Everlasting. Slumber seizes Him not, neither sleep; to Him belongs all that is in the heavens and the earth. Who is there that shall intercede with Him save by His leave? He knows what lies before them and what is after them, and they comprehend not anything of His knowledge save such as He wills. His throne comprises the heavens and earth; the preserving of them oppresses Him not; He is the All-High, the All-Glorious." (Holy Qur'an, Chapter 2, verse 255)
Allah (swt) said: "People of the Book, go not beyond the bounds in your religion, and say not as to God but the truth. The Messiah, Jesus son of Mary, was only the Messenger of God, and His Word that He committed to Mary, and a Spirit from Him. So believe in God and His Messengers, and say not, 'Three.' Refrain; better is it for you. God is only one God. Glory be to Him - (He is) above having a son." (Holy Qur'an, Chapter 4, verse 171)
Allah (swt) said: "Act equitably; Indeed, Allah loves the equitable." (Holy Qur'an, Chapter 49, verse 9)
Allah (swt) said: "Truly, Allah loves those who repent, and loves those who care for cleanliness." (Holy Qur'an, Chapter 2, verse 222)
Allah (swt) said: "In the name of Allah, The Beneficent, The Merciful." (Holy Qur'an, Chapter 1, verse 1)
Allah (swt) said: "My Mercy has embraced all things." (Holy Qur'an, Chapter 7, verse 156)
Allah (swt) said: "Allah is better [than any one] at guarding, and He is The Most Merciful of those who show mercy." (Holy Qur'an, Chapter 12, verse 64)