Romans used wreaths as a symbol of victory. This quite possibly was the start of using wreaths on the doors.
There is also the Advent wreath that started with folk practices involving the Germanic people before Christian times. In the cold December (or that time of year) would use evergreen wreaths and fires to symbolize the hope of the approaching spring and the renewing of light. As time went on Christians practiced these ideas too.
In the 16th century, these symbols started to be meant for the Advent of Christ and him being the everlasting light. Catholics and Protestants both used the wreaths of evergreen and the fire.
The Advent wreath then started to spread from Germany to the rest of the Christian World. Instead of lighting fires people light candles in an Advent wreath. The usual wreath has 4 candles around the evergreen wreath and sometimes a 5th candle is in the middle of the evergreen wreath. In the traditional Advent wreath 3 of the 4 candles are violet, the 4th is rose, and the 5th candle (when used) is usually white. The violet symbolizes faithful expectation. Rose symbolizes hop and joy. The white candle is for Christ's birth. One candle a week is lit starting the 1st week in December. On Christmas Day or sometime Christmas Eve the white candle gets lit.
In the 19th century an American tradition utilized boughs and greenery were used for making Christmas wreaths and used for honoring those that had died at Christmas. The wreaths were made by weaving the boughs together with stars and crosses and placed on the graves of the dead loved ones. Then they would be taken home by the families and used to decorate the house. Then the artificial greenery appeared on the scene with plastic evergreen and holly. Wreaths started being made artificially by some people so they could be kept from year to year unlike the natural ones.
Traditional Christmas wreaths in religious groups are true symbols of one's faith. The Christmas wreath is a symbol of the eternity of Gad because it is a circle that has not start or finish point. They also are a sign of God's mercy. By using the leaves of evergreens and or the branches the Christmas wreaths also symbolic of everlasting life and of God's everlasting love. New life and hope is symbolized in the color green.
Today, Christmas wreaths are used as symbols of the holiday season. They decorate our front doors, streets, stores, and homes. Many businesses hang Christmas wreaths or sometimes now they are called holiday wreaths. The Christian churches use wreaths to decorate at Christmas too.
Even non-Christians use Christmas wreaths to adorn their houses and celebrate the more commercial part of the season.
One great family Christmas activity these days is to buy the greenery with real or artificial and make a Christmas wreath. All sorts of decorations can be bought. The family laughs together as they decorate the wreath to hang on their door.