The Advent Wreath Explained
The wreath is a circle to symbolize the never ending cycle.
Evergreen sprigs are used to symbolize eternity.
The four candles represent the four weeks and themes of advent. Each is lit on its week and if its week has past. So, on week one only the first candle is lit. On week two, both the first and second candles are lit.
The first candle, the candle of hope, also known as the prophecy candle, is lit to remind of us that we anticipate the second coming of Christ. We wait in hope.
The second candle, the candle of peace, also known as the Bethlehem candle, is lit to remind us of the humility and lowliness of Christ's first coming. The birth in the manger brought peace.
The third candle, the candle of joy, also known as the shepherds' candle, is lit to remind us of the joy of knowing of the birth of Christ. The shepherds were filled with joy knowing that the Christ was born.
The fourth candle, the candle of love, also known as the angels' candle, is lit to remind us that even the heavenly angels are aware of the love of God and that the birth of Christ was world changing.
And the center candle (if one is present), called the Christ candle, is lit on Christmas Eve or Christmas day. This represents the presence of Christ in the world. In churches around the world, the Christ candle is lit at the beginning of Sunday worship to symbolize His presence in the sanctuary. That same candle is removed from the sanctuary during a Maundy Thursday service, representing the death of Christ on the cross. Easter morning a new Christ candle is lit and the sanctuary is bright with the light of Christ.
Colors of the Candles
There are three main traditions. In the Anglican tradition all four candles are red. In the other two traditions either all the candles are purple or royal blue (both colors of royalty as this is about Christ). The only other deviation is if the third, and only the third, candle is pink. In some western traditions, the third Sunday of the Advent season is also called Gaudete Sunday. Gaudete is a Latin word meaning rejoice. It is the first word from the song sung at the beginning of the Eucharist service on that day.
Link to Wikipedia and the History of the Wreath