I am not ashamed to borrow someone else's answer. Of all answers to the question i(n the field of theology), "What is God?", I like Paul Tillich's anwer, "The ground of being." God is the ultimate reality upon which everything in the universe is created.
Examples and analogies:
1) If there is no 'ground,' we can't stand up.
2) God represents the universe itself, and God embraces everything inside the universe.
3) God is the totality of all things, and God's creations are found within this totality.
4) God is the reason that you and everyone else (myself included) can live in this universe.
Is God Jesus' daddy? Of course, yes, if you're a Christian. Did God reveal Himself to Mohammad (Peace be upon him)? Of course, yes, if you're a Muslim. Did God speak through various Jewish prophets as written in the Hebrew Bible? Of course yes, if you're a person of Jewish faith.