Baptism In The Spirit
If water baptism means a full immersion into living for Jesus through water, then Baptism in the Spirit means a full immersion into the fullness of a life guided by Holy Spirit.
Who is this 'Holy Spirit'? Well, he's a person. We use a term the 'trinity' to describe God - God, the Father - God, the Son - and God, the Holy Spirit - He's the one who guides us how to follow Jesus, helps us understand God's will, gives us guidance as to what to do or say in a given situation.
He doesn't force you into anything and it's always for the good of others. So this baptism is letting him come and occupy the space in your life that's been specially made for him.
He's the one who does miracles, answers prayer and helps you know that God is there. Some of the more unusual things (that is until they are a part of your every day life) happen through the Holy Spirit - he tells you things you didn't know, gives you the power to heal people in Jesus name, helps you praise God with a heavenly language (speaking in tongues) - helps you understand if something is of God or is downright evil.
It's through the Holy Spirit that we also learn to worship and praise God - how to do it with reality and meaning.
And unlike water baptism it's not a once and for all experience but a daily and continual 'filling' and 'refilling' a continual 'baptism' or immersion that refreshes our everyday lives.
Acts 1:8
'But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.'