God is just a generic word that we use today to describe a supreme being. The word God is not the name of God. Unfortunately it has become synonymous with Christianity. But all religions have a god. And they are actually all the same god BUT only in existence not in practicality. Such as Muslims call God, Allah. Yes, in existence it is the same God. They believe that their people whom came from Ishmael are the true believers. Their writings are different. Their customs are different and their practices are different. Therefore their religion becomes different and in the end their god becomes false.
If you notice the name Allah is very similar to that name which Jesus cried out while on the cross, Eloi, which is Aramaic, a family language to that of Hebrew and Arabic. The generic term for God in Hebrew is El, although it is not so generic because unlike many of us we (as Christians) do not use the term God in as much respect as they do. To know which god are false and views of God by different religions follow this link: Views of God.
A. Who is God?
God is the One who is all. He is the supreme being. Without Him none of us or anything would be here. No one one knows His real name. Most Hebrew scholars will tell you that God's real name is unpronounceable. The name though given to the Jews in Exodus is Yahweh, which is translated as "I AM". He has always been here and always will be. He encompasses all that is. Now given that He does have a few titles/names that He goes by that we call Him. Just like if we have a special person in our life we have a few things we call them depending upon our needs. See Names of God.
B. Where is God?
Everywhere. Now that is not to say He is like pagan gods and is in the rocks and the water. But as His name declares--He is. All that we know is Him. This is a hard concept for most. How can something be everywhere? It's easy if you are not part of our space and time. It's easy if you are not just physical matter. It's easy if you are more than just a form of energy.
C. When did God come into existence?
He has always been here. This is the hardest concept for us to understand. Yet, infinity is something we all know. Start counting. When do you stop? You can't. How far does space go? Some say forever (although it doesn't) and yet they can't understand the existence of God. Again, as His name tells us, I AM, explains that He has always been. Remember we are four dimensional beings. We are limited in time and space. He is not.
D. What does God do?
He does all. But that doesn't show you what He does. Some will disagree but God manifests Himself to the human race as three parts. It's a simple concept here as well. Humans have a body that we interact with the world. We have a mind that we can interact with other minds. And we have a spirit that we can interact with God. See Body, Soul, Spirit for more. God is infinite. But so man can understand His existence He manifests Himself as God the father, God the Son, and God the Holy Spirit.
He is much more than just this but this is accurate and appropriate to view God in this way. God works in threes. Faith, hope, love as example all work together. Below is a chart, NOT TO SHOW HIERARCHIC POWER, but to give a glimpse of His names and how He manifests Himself to us.
The above is not an exhaustive list. There are even more names and titles for God in the Bible. This is just to show some of the capacity that God has. As YHWH is the Creator of all things. But in the beginning of time (for man) YHWH calls Himself Elohim. "El" meaning God. "Im" meaning plurality. "H" meaning breath and grace. Elohim basically means out of the grace of God He made man in His image. And notice in Genesis that it says "OUR" image. This shows the infinity of God and explains that the Father, Son, and Spirit were there as one.
The names in the second line is what Jewish people would call the three with translation underneath. Notice if you put the father, the son, and the spirit together you get the name of the savior is holy! Jesus is the name of the Savior. His name is holy and is above all other names. Now that is just a play on words and a coincidence but it worked out nicely.
The third tier are titles given to each part of the Trinity, again not an exhaustive list. The ones for the Father show power, authority. He heals us. He provides for us. He is the most high. He has all power. He gave us life. He is our banner. He is the host of heaven. The ones for the Son show how He finishes God's plan and how He has been given authority. He is the King of Kings. He was the messenger of God. Through Him we are saved. He had become the atonement for our sin by being anointed by God. He came here to be with us and is the first and last of all things. He became manifested in the body of man named Jesus. The name that has been given authority over every other name. The ones for the Spirit show that He will never leave us. He is here to guide us and give us wisdom in a world of stupidity. He comforts us in our time of need and He grants us gifts of the Spirit to help us discern issues and to help glorify who God is.
But above all, remember God is One. All the above is one Being. Just like you have many parts and emotions and even roles you have in your life, you still are one person, the same it is with God; only He is greater than we are.