" I saw the Spirit come down like a dove from heaven and stay on him. I still did not know that he was the one, but God, who sent me to baptize with water, had said to me ' You will see the Spirit come down and stay on a man; he is the one who baptizes with the Holy Spirit. 'I have seen it," said John, " and I tell you that he is the Son of God ." (John 1 )

Please explain what is "Holy Spirit " ?
Why " Spirit come down like a dove " ?
And normally baptize with what ?

Thanks !