What is the Holy Ghost?

This concept doesn't make sense to me. I've read books from the bible, but I don't remember reading anything about the Holy Spirit, or at least saying that it is a separate person. What does he look like? The holy trinity is 3 people right? So who is the third guy? He can't just be the manifestation of God's presence on earth cause that's still just God. Can someone please explain to me how this makes sense?