The Advent wreath is a traditional Catholic devotion that was primarily for use in the home. The Advent wreath kit that my family had when I was growing up came with prayers and instructions that assumed it would be used at the family dinner table.
The circular shape of the wreath, and the evergreens out of which it is made, or with which it is decorated, symbolize eternity. The four candles, of course, symbolize the four weeks of Advent; the light of the candles symbolizes the coming of Christ, the Light of the World. Traditionally, three of the four candles are purple, while the third is pink; if the wreath is decorated with ribbon, it is also purple. Purple is the liturgical color for Advent: it is the color of royalty, and the color of penance.
Prior to Vatican II, Advent was celebrated in a more penitential manner than it is today; part of the liturgical reform was to make a greater distinction between the anticipation of Advent, and the repentance of Lent.
The third candle is pink, which is also the liturgical color for the third Sunday of Advent, which is called Gaudete Sunday. Gaudete is Latin for “Rejoice” (or, more precisely, “Rejoice, you!”), and comes from the first word of the opening antiphon in the missal for this Sunday. On the third Sunday of Advent, we rejoice because we are more than halfway to Christmas.
My other blog is named after Gaudete Sunday.