Jesus talks a lot about the Holy Spirit, or what in Greek is called the Paraclete in His Farewell Discourse of chapters 14-17. (This comes from parakletos, which means "called" - kletos and "alongside" - para. Attorneys, or counselors, were "called alongside" to defend and counsel.)

The prominence of the Paraclete addresses a key set of theological questions:

1) If Jesus as the incarnate Word of God brings a distinctive revelation of God to the community, what happens when the incarnation ends?
2) Was the revelation of God in Jesus available only for those who had firsthand experience of the historical Jesus and His ministry?
3) Is Jesus’ revelation of God limited to one moment in history, or does it have a future beyond its particular historical moment?

In the Farewell Discourse, Jesus reveals the Paraclete as the link between His ministry in that moment of history and the future life of the church after Jesus’ death. What does the Paraclete, or the Holy Spirit, do? Jesus reveals several roles (these come from our Disciple IV manual, with my observations in [ ]):

1) The Paraclete will come only after Jesus leaves. The Paraclete’s coming makes Jesus’ going advantageous to the believer. (John 16:7)
2) The Paraclete will come from the Father in Jesus’ name. As the Father sent Jesus, Jesus sends the Spirit. (John 14:6; 15:26)
3) The Paraclete will say what Jesus wants Him to say just as Jesus has said what the Father wanted Him to say. (John 16:13-15)
4) The Paraclete will help disciples remember all Jesus said and did, "remind you of all that I have said to you." (John 14:26)
5) The Paraclete will teach and explain new things that disciples prior to the Crucifixion and Resurrection were not able to understand. Jesus still has much to teach us. (John 16:12) [John’s Gospel itself is evidence of this. It is interpretation of the events of Jesus’ lifetime, written after some 60 years of reflection and living with the Paraclete in the church. This truth about the Paraclete’s new teaching would later cause trouble, however, as people arose in the 2nd century claiming direct revelation from Jesus that was contrary to orthodox teaching, e.g., the Gnostics. It’s still an issue.]
6) The Paraclete will give us peace, not someday but now. As Jesus said to the disciples in the boat, so the Spirit will say, "Do not be afraid." (John 6:20; 14:26-27)
7) The Paraclete will convict the world of its sin and show the world righteousness as Jesus did. Wherever Jesus’ Spirit is present, judgment takes place, revealing light and darkness, life and death. (John 16:8-11)
8) The Paraclete is the Spirit of Jesus, the Spirit of Truth. Jesus is the Truth. The Holy Spirit will say and do only what Jesus would say and do. (John 16:14) [It’s interesting to note that Luke in the Book of Acts uses the terms "Holy Spirit" and "Spirit of Jesus" interchangeably.]

The Holy Spirit is the least understood person of the Trinity. We do well to understand what John reveals to us about the Paraclete.