Holy Spirit in Mormonism
Like LDS teachings on God the Father aren't the same as orthodox Christianity, its teachings of Jesus Christ aren't the same as orthodox Christianity, neither are its teachings on the Holy Spirit. While there are some overlapping ideas, there are significant differences, enough to make LDS doctrine original and unique.
When teenage Joseph Smith told people about the special revelations he received from God in the 1820's (pictured right), some concepts were similar to orthodox Christianity, yet other ideas were new because, according to his testimony, all existing Christian churches at the time were wrong.
Since the LDS church teaches that is the true restoration of the New Testament church, comparing Mormon and Christian beliefs and practices is an important exercise for understanding (click link to see a comparison chart).
Who is the Holy Spirit in LDS thinking?
He is a comforter
According to Mormon beliefs, the Holy Spirit, who is commonly called the " Holy Ghost," is the third person of the Godhead whose special mission is to guide and teach, testify to God, and comfort and sanctify human souls.
In these affirmations, Mormonism is in agreement with mainstream Christianity.
He is a separate being from the Father and son
Mormonism departs from mainstream Christianity, however, in teaching that the Holy Spirit is a "one in purpose with the Father and the Son, but is a separate being"  rather than an aspect of God or part of a Trinity.
He is pure spirit
Unlike heavenly father and Jesus, who have physical bodies, the Holy Spirit is pure spirit and has no body. Joseph Smith wrote that God revealed to him that "the Holy Ghost has not a body of flesh and bones, but is a personage of Spirit. Were it not so, the Holy Ghost could not dwell in us." 
- "Who is the Holy Ghost?" Mormon.org.
- Doctrine and Covenants 130:22