Q: "Who or what is the Holy Spirit? I've seen this name/phrase on your site in a few places."
our A: The Holy Spirit is a real person who came to reside within Jesus Christ's true followers after Jesus rose from the dead and ascended to heaven (Acts 2). Jesus told His apostles...
"I will ask the Father, and He will give you another Helper, that He may be with you forever; the Spirit of truth, whom the world cannot receive, because it does not behold Him or know Him, but you know Him because He abides with you, and will be in you. I will not leave you as orphans; I will come to you." (John 14:16-18)
The Holy Spirit is not a vague, ethereal shadow, nor an impersonal force. He is a person equal in every way with God the Father and God the Son. He is considered to be the third member of the Godhead. Jesus said to His apostles...
"All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age." (Matt. 28:18-20)
God is Father, Son and Holy Spirit. And all the divine attributes ascribed to the Father and the Son are equally ascribed to the Holy Spirit. When a person becomes born again by believing and receiving Jesus Christ (John 1:12-13; John 3:3-21), God resides in that person through the Holy Spirit (1Cor. 3:16). The Holy Spirit has intellect (1Cor. 2:11), emotion (Rom. 15:30), and will (1Cor. 12:11).
A primary role of the Holy Spirit is that He bears "witness" of Jesus Christ (John 15:26, 16:14). He tells people's hearts about the truth of Jesus Christ. The Holy Spirit also acts as a Christian's teacher (1Cor. 2:9-14). He reveals God's will and God's truth to a Christian. Jesus told His disciples...
"The Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you." (John 14:26)
"When He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come." (John 16:13)
The Holy Spirit was given to live inside those who believe in Jesus, in order to produce God's character in the life of a believer. In a way that we cannot do on our own, the Holy Spirit will build into our lives love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control (Gal. 5:22-23). Rather than trying to be loving, patient, kind, God asks us to rely on Him to produce these qualities in our lives. Thus Christians are told to walk in the Spirit (Gal. 5:25) and be filled with the Spirit (Eph. 5:18). And the Holy Spirit empowers Christians to perform ministerial duties that promote spiritual growth among Christians (Rom. 12; 1Cor. 12; Eph. 4).
The Holy Spirit also performs a function for non-Christians as well. He convicts people's hearts of God's truth concerning how sinful we are -- needing God's forgiveness; how righteous Jesus is -- He died in our place, for our sins; and God's eventual judgment of the world and those who do not know Him (John 16:8-11). The Holy Spirit tugs on our hearts and minds, asking us to repent and turn to God for forgiveness and a new life.