The Christmas wreath is circular, no beginning and no end, so it symbolizes eternity.  Traditionally, it is made from holly, pine, or other evergreen plants.  The evergreens symbolize eternal life.

The wreath has an ancient history.  The Greeks and Romans crowned heroes, athletes and kings with wreaths of laurel, holly, or olive branches.  Aristocrats began adding precious jewels eventually creating crowns.

And, of course, Jesus was crowned with a wreath of thorns as an insult by the Roman soldiers.