At the time appointed by God, the only Son of the Father, the eternal Word, that is, the Word and substantial Image of the Father, became incarnate; without losing his divine nature he has assumed human nature.
Jesus Christ is true God and true man, in the unity of his divine person; for this reason he is the one and only mediator between God and men.
Jesus Christ possesses two natures, one divine and the other human, not confused, but united in the one person of God's Son.
Christ, being true God and true man, has a human intellect and will, perfectly attuned and subject to his divine intellect and divine will, which he has in common with the Father and the Holy Spirit.
The Incarnation is therefore the mystery of the wonderful union of the divine and human natures in the one person of the Word.
- Paras 479 to 453 of The Catechism of the Catholic Church
For a fuller explanation of this mystery, go to this commentary on the Summa Theologica of St Thomas Aquinas.