The Greek word Theophany means an appearance of God. The word Epiphany means a ‘shining forth’. In this case both of these words refer to today’s feast. It commemorates the Baptism of Jesus Christ in the Jordan by St. John the Baptist. This begins the public ministry of Jesus Christ. It is a manifestation of God because Jesus Christ, the Son of God, begins his ministry by coming for baptism, God the Father reveals Himself through his voice from heaven and the Holy Spirit appears in the form of a dove.
(Mark 1:9-11) In those days Jesus came from Nazareth of Galilee and was baptized by John in the Jordan.  And when he came up out of the water, immediately he saw the heavens opened and the Spirit descending upon him like a dove;  and a voice came from heaven, “Thou art my beloved Son; with thee I am well pleased.”
Incidentally, the emphasis on the ‘shining forth’ of God shows that Epiphany is the fulfillment of the Jewish feast of Light (Hanukkah) just as Pascha is the fulfillment of Passover and Pentecost is the fulfillment of the Jewish feast of Weeks (Shavuot). Also, in the ancient church there was the custom to baptize people at the Vespers of heophany. This indicated that baptism is the illumination of the human race. This feast goes back to the early church. In the 2nd century, St. Clement of Alexandria writes about the vigil celebrated on the eve of the feast. From the 3rd century we have a dialogue about the feast from St. Gregory the Wonderworker and the Martyr St. Hippolytus. From the 4th to the 9th century many of the great Fathers of the church wrote about the feast of Theophany.
On the eve of the feast texts from the prophet Isaiah about the coming of the Messiah and his Forerunner John the Baptist are read: (Luke 3:4-6) As it is written in the book of the words of Isaiah the prophet, “The voice of one crying in the wilderness: Prepare the way of the Lord, make his paths straight. 
Every valley shall be filled, and every mountain and hill shall be brought low, and the crooked shall be made straight, and the rough ways shall be made smooth; and all flesh shall see the salvation of God.”
On the day of the feast at the Divine Liturgy the usual “Holy God, Holy Mighty, Holy Immortal..” is replaced by a passage from St. Paul’s letter to the Galatians, “… as many of you as were baptized into Christ have put on Christ” (Gal 3:27). This reminds us that in the early church people were baptized on the feast after a period of instruction and it reminds us that our baptism is not simply an event of the past, but rather our “putting on Christ” has changed us and the grace of baptism always remains in us. The Gospel tells us of Christ’s baptism.
Perhaps the most memorable of the aspects of the feast of Theophany is the Great Blessing of Water. In our parish we bless the font filled with water in the church, then on the next available Sunday we process to the harbor to bless water there. The water stands for the world, good as God created it, but now blighted by sin, the world which will be glorified when Jesus comes at the end of time to “make all things new”.
Here are passages from the service of the blessing of water:
The voice of the Lord upon the waters cried saying: “Come, receive the Spirit of wisdom, the Spirit of understanding, the Spirit of the fear of God, from Christ Who is made manifest.
Today the nature of the waters is sanctified. The Jordan is parted in two; it holds back the streams of its own waters seeing the Master wash Himself.
Before the blessing of water the reader reads three passages from the book of Isaiah which refer to the water. For example Isaiah 55:1: “Ho, everyone who thirsts, come to the waters; and he who has no money, come, buy and eat! Come, buy wine and milk without money and without price.”
The Epistle tells about baptism and the Gospel tells us about Jesus’ baptism in the Jordan. A Great Litany is then chanted asking God to send the Holy Spirit to bless the water and through it the entire material creation. After the dipping of the cross and its raising up the priest blesses the church and the people. After this service the priest then visits the homes of the parishioners to bless them with water which was blessed in church.
Finally, as mentioned above, this is no mere symbolic ritual. Through the descent of the Holy Spirit into the water we participate in the renewal of the cosmos beginning with the countless blessings of water throughout the Orthodox world, which begins here and now and which links us to that renewal of all things at the end of time.
Troparion — Tone 1
When Thou, O Lord wast baptized in the Jordan the worship of the Trinity was made manifest for the voice of the Father bore witness to Thee and called Thee His beloved Son. And the Spirit, in the form of a dove, confirmed the truthfulness of His word. O Christ, our God, who hast revealed Thyself and have enlightened the world, glory to Thee!
Kontakion — Tone 4
Today Thou haste shown forth to the world, O Lord, and the light of Thy countenance has been marked on us. Knowing Thee, we sing Thy praises. Thou hast come and revealed Thyself, O unapproachable Light.