The Holy Ghost is God and the Third Person of the Blessed Trinity.
The Holy Ghost proceeds from the Father and the Son. This does not mean that the Holy Ghost began to exist later in time than the Father and the Son. He proceeded from them from all eternity. He is to them what warmth is to fire, existing and proceeding at the same time. There can be no fire without warmth; if there was an eternal fire, there would be an eternal warmth. So, since there are the eternal Father and Son, there is the eternal Holy Ghost. The Holy Ghost is the eternal, mutual love that the Father and Son have for each other; but instead of being mere feeling, He is a Person, a Being – God.
The Holy Ghost is equal to the Father and the Son. True God just like the Father and the Son, the Holy Ghost is eternal, all-knowing and almighty. The Third Person is called Holy Spirit, from the Latin word “spiritus”, a breath. He was breathed forth by the Father and the Son. In English we also call Him Holy Ghost. Other names used to refer to the Holy Ghost are: Advocate, Paraclete, Consoler, Comforter, Substantial Love, Spirit of Truth etc.