Tomorrow, millions of Christians around the world will celebrate the Day of Pentecost, marking the occasion almost 2,000 years ago when God sent His Holy Spirit to live in us. (You can read about that momentous day in the New Testament, in the second chapter of the book of Acts.)
Who is the Holy Spirit? The Holy Spirit is God Himself, as He lives within us and also works in the world. Just as Jesus was fully God (as well as fully man), so the Holy Spirit also is fully God. (This, incidentally, is why we shouldn't refer to the Holy Spirit as "it," but as "Him" - for He is a Person, the third person of the Trinity.)
Shortly before He ended His earthly ministry, Jesus promised His followers that the Holy Spirit would come to take His place: "I will ask the Father, and he will give you another Counselor to be with you forever" (John 14:16). Why did He come? He came to help us - because we can't live the way we should by ourselves. He also came to guide us - because we need to know God's will. He also came to assure us of God's presence - because we need God's comfort.
If you have given your life to Jesus Christ, God's Holy Spirit now dwells within you. Thank God for this truth, and yield your life to His power every day, because "the Spirit helps us in our weakness" (Romans 8:26).