In most Christian churches, the Holy Spirit, or Holy Ghost, is the third person of the Trinity. The Trinity is a God made up of three persons: God the Father, God the Son, and God the Holy Spirit, who share the same qualities and essence and are therefore One God.[1] Christians who don't believe in the Trinity have a big difference in their beliefs about the Holy Spirit.
In Christian theology, pneumatology is the study of the Holy Spirit.
Christianity came from Judaism. People who study the Bible often identify the Holy Spirit with the the Ruach Hakodesh in Jewish scripture. They think that Jesus was expanding upon these Jewish concepts. [2]