What Is the Holy Spirit?

How is the Holy Spirit described in Scripture?

2 Timothy 1:7
God has not given us a spirit of fear, but of power and of love and of a sound mind.
Zechariah 4:6
So he answered and said to me: "This is the word of the Lord to Zerubbabel: ‘Not by might nor by power, but by My Spirit,' says the Lord of hosts."
Micah 3:8
"But truly I am full of power by the Spirit of the Lord, and of justice and might, to declare to Jacob his transgression and to Israel his sin."
Luke 4:14
Then Jesus returned in the power of the Spirit to Galilee, and news of Him went out through all the surrounding region.
Acts 1:8
"But you shall receive power when the Holy Spirit has come upon you; and you shall be witnesses to Me in Jerusalem, and in all Judea and Samaria, and to the end of the earth."

The Holy Spirit is often described as the power of God. As our Fundamental Beliefs booklet explains, "The Holy Spirit of God is not identified as a third person in a trinity but is presented in Scripture as the power of God, the mind of God and the very essence and life force of God through which the Father begets human beings as His spiritual children." There is much more to this subject, so in addition to the link above, please also see "Is the Holy Spirit a Person?" and "Why Can't Theologians Explain the Trinity Doctrine?"
Receiving the Holy Spirit gives the Christian the spiritual power needed to live a Christian life. It helps provide the motivation and strength that make it possible for us to be able to obey God's commandments, make right choices, overcome and persevere (Romans 8:26; 12:2; Philippians 2:5; 2 Peter 1:3-4).