Allah is simply the Arabic word for God. Arabic-speaking Christians and Jews use the same word in their prayers and Bibles. Muslims believe Allah is the same God who spoke to Abraham, Moses and Jesus.
At the heart of Islam is tawhid: the belief that God is One. He has no partners, no children and no equals. He is not part of creation, yet He is closer to us than our own jugular vein. A short chapter of the Qur’an sums it up:
قُلۡ هُوَ ٱللَّهُ أَحَدٌ ١ ٱللَّهُ ٱلصَّمَدُ ٢ لَمۡ يَلِدۡ وَلَمۡ يُولَدۡ ٣ وَلَمۡ يَكُن لَّهُۥ كُفُوًا أَحَدُۢ ٤
Say, "He is Allah, [who is] One Allah, the Eternal Refuge He neither begets nor is born Nor is there to Him any equivalent
Qur'an 112:1-4
Muslims come to know God through His names. The two most repeated are Ar-Rahman and Ar-Rahim, the Entirely Merciful and the Especially Merciful. Every chapter of the Qur’an but one begins with them. You can explore all 99 Names of Allah.