The Holy Spirit, or Holy Ghost, is God, the third Person of the Holy Trinity, who, from eternity, "proceeds" from the Father (John 15:26). The Holy Spirit is co-equal with the Father and the Son.
The Spirit dwells inside every true Christian, each one's body being his temple (First Epistle to the Corinthians 3:16). He is depicted as a "Counsellor" or "Helper,", guiding them in the way of the truth. The "Fruit of the Spirit" (i.e. the result of his work) is "love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control" (Galatians 5:22).
Christians believe that it was the Holy Spirit whom Jesus mentioned as the promised "Comforter" (i.e. "strengthener", "fortifier") in John 14:26. After his resurrection, Christ told his disciples that they would be "baptized with the Holy Spirit" (Acts 1:4-8); a promise that was fulfilled in the first Pentecost.