Allah

Allah is God, the one and only Creator & Sustainer of the universe. Allah creates human beings, animals, plants, mountains, rivers, oceans, etc. Allah judges the deeds (actions) of human beings, punishes individuals for their bad deeds and rewards them for their good deeds, both in their life on Earth and on the Day of Judgement.
Allah (God) sent prophets (such as Noah, Abraham, Moses, Jesus, and Mohammad, peace be upon them) to call people to follow God's religion. The Quran prescribes for Muslims to regard all prophets of God as equally worthy of respect.
Some Christians think that Mohammad is Allah or that Muslims worship Mohammad (p). This is 100% false. This erroneous belief of Christians stems from the fact that Christians worship Jesus, so they falsely presume that Muslims worship Mohammad (p).
Mohammad (p) was simply a human prophet. He never claimed to be God. Muslims do not worship prophet Mohammad, directly or indirectly. Islam is  a strictly Monotheistic religion. Islam highly emphasizes that there is only one God. No human being has divine attributes. God's prophets are not divine, but they are assigned a mission by God to call people to follow God's religion.
Muslims refer to God as Allah, Ellah, or Allahom (meaning "the God" in Arabic). The word "Allah" in Arabic means "the god" or "God". In Arabic, the word Allah is derived from of a contraction/ abbreviation of two words: (1) "Al" (also can be pronounced as El) which mean "The", and (2) "Elah" which means "god".
Islam did not invent or introduce the word Allah to the World:
   - The Arabic Bible that has been used by Arab Christians, for the past 2000 years since the time Jesus, refers to God as Allah.

   - The Hebrew Bible refers to God as Elohim which is similar to Allah/ Ellah/ Allahom used by Muslims.

   - The word that corresponds to Allah in Aramaic language (the mother tongue of Jesus), and used in the Aramaic Bible, is “Elaha”.

 - The word that corresponds to Allah in Syriac language, and used in the Syriac Bible, is “Alaha”.

Father Dr. Labib Kobti, an Arab Christian priest, in one his articles titled The Christian Arab Heritage (available through this link http://www.al-bushra.org/arbhrtg/arbxtn04.htm ) says: "Recently Father Pecerillo, a famous Franciscan Archaeologist, found more than twenty churches in Madaba at the south of Jordan. From the Fourth Century, we found houses in Syria, Lebanon, Iraq and Palestine with this inscription in Arabic :"Bism El-Lah al Rahman al Rahim" that showed that Christians were the first to use this name so as to indicate their belief in the Holy Trinity, more than two hundred years before Islam." 
What this Christian priest is revealing is extremely important for the following reasons:
(1) This Arabic phrase "Bism El-Lah al-Rahman al-Rahim" ( بسم الله الرحمن الرحيم ) that the priest says has been discovered in houses of Christians dating back to the 4th Century is the same phrase that most Suras (Chapters) of the Quran start with. This phrase can be translated as "In the name of Allah the Most Compassionate, Most Merciful".
(2) The fact that this phrase was found in houses of Christians dating back to the 4th Century, meaning three centuries before the Quran was revealed to Prophet Mohammad. This indicates that the beliefs of early Christians were very similar to the beliefs of Muslims. In other words, early Christians did not believe in the Trinity: "In the name of Father, Son, Holy Spirit", instead they used the Quranic phrase "In the name of Allah the Most Compassionate, Most Merciful". This is proof that Jesus was a Muslim and he preached Islam, and early followers of Jesus were Muslims.