What is the Holy Spirit?

Here is one definition from the Catholic Encyclpedia:

The doctrine of the Catholic Church concerning the Holy Ghost forms an integral part of her teaching on the mystery of the Holy Trinity, of which St. Augustine (De Trin., I, iii, 5), speaking with diffidence, says: "In no other subject is the danger of erring so great, or the progress so difficult, or the fruit of a careful study so appreciable". The essential points of the dogma may be resumed in the following propositions:

• The Holy Ghost is the Third Person of the Blessed Trinity.
• Though really distinct, as a Person, from the Father and the Son, He is consubstantial with Them; being God like Them, He possesses with Them one and the same Divine Essence or Nature.
• He proceeds, not by way of generation, but by way of spiration, from the Father and the Son together, as from a single principle.

And from Wikipedia, the free onling Encyclopedia:

In various religions, most notably Christianity, the Holy Spirit (also called the Holy Ghost in Trinitarian Christianity; in Hebrew רוח הקודש Ruah haqodesh) is the third Person of the Holy Trinity. As such, the various Christian perspectives view him as God himself, a form of God, or a manifestation of God. The word "Spirit" commonly translates the Greek New Testament word pneuma (Greek: πνεύμα).

And for even more of the formal, official or doctrinal defintions.

Are you sufficiently confused? If not, you can stop reading right here. On the other hand, if you are interested in learning more about the Holy Spirit, its role in our lives, in the church and in the world at large, you might continue exploring the following:

The Holy Spirit: powerful and ever-present, but poorly understood.

For more about the Holy Spirit

For more about the Trinity

All About Pentecost

For more about Pentecostalism

What is Glossolalia / Speaking in Tongues?

For the story of Pentecost from Acts 2

For a meditation on Pentecost