Who is the Holy Spirit?
The obvious way to answer this question would be to say that the Holy Spirit is the 3rd Person of the Trinity. So what is the Trinity? Please click this link if you would like to read more about the Trinity. Think of God the Father as the God above us, Jesus (God the Son) as the God among us and God the Holy Spirit as the God within us.
Being fully human and being only one human, Jesus could only do so much in his lifetime to spread the his teachings. Even the disciples didn’t really start understanding what Jesus was trying to say until he was crucified and raised from the dead. But Jesus told them that though he had to leave them… it was good that he would leave them because then he could send his friend, the comforter or advocate.
John 16:7New International Version (NIV)
7 But very truly I tell you, it is for your good that I am going away. Unless I go away, the Advocate will not come to you; but if I go, I will send him to you.
It makes perfect sense. Jesus, the God among us had to fulfill his purpose by going to the cross and he then sent the Holy Spirit so that he(God) could be present in each of us anywhere all the time. The Holy Spirit lives within us guiding us and helping us.