Silly children. There is no simple explanation of what the Spirit is, other than to say he is that part of God that comes to each person individually. It is much easier to talk about what the Spirit does, than what he is, and even that is not particularly easy.
There is a clear distinction between the Spirit as described in the Old Testament and the one described in the New Testament. In the OT the Spirit of God, literally the Breath of God, was a gift that came, bestowed and left.
In the NT the Spirit is refered to as a person, most often called the Comforter, in addition to his name. This Spirit is indwelling and was visible on the first Pentacost as a tongue of fire appearing over the heads of the Apostles. And tongues spreading out like a fire appeared to them and came to rest on each one of them. All of them were filled with the Holy Spirit, and they began to speak in other languages as the Spirit enabled them. Acts 2: 3,4. From that point onward the 11 surviving Apostles were energized and empowered after a month and a half of living in fear of reprisals from the Jewish leaders that had arranged Jesus' execution, especilly since Jesus body was missing and the disciples weree being blamed for stealing it (Not that this made much sense. There was an elite Roman guard tasked to preventing that very event, but that's another discussion).
For all of the Spirit's importance, there is surprisingly little written about it. Mostly it is refered to in passages like the one quoted to explain the force of a speaker, or as the manifestation or conduit of God's action in a person's life, eg In the same way, the Spirit helps us in our weakness, for we do not know how we should pray, but the Spirit himself intercedes for us with inexpressible groanings. And he who searches our hearts knows the mind of the Spirit, because the Spirit intercedes on behalf of the saints according to Gods will.Romans 8: 26,27.
In Christian theology, Man is powerless to change his condition. The Spirit is the avenue which God uses to reach out and touch each individual person, so that in the end some will find the way. He is our personal guide to heaven if you will.
J