Because evergreen plants don't lose their leaves in winter, they have long been symbols of immortality.  They can also symbolize youth, vigor, longevity, and generative power.  Wreaths of various types of evergreens are used at Christmas representing immortal life through Christ.  The evergreen Christmas tree is a symbol of rebirth.  In many ancient cultures and religions, some evergreen plants or branches of evergreen trees were brought indoors as a symbol of rebirth and immortality.  
In addition to the symbology of evergreens in general, some specific evergreens have additions symbology:

• Cedar - symbolizes strength, nobility and incorruptibility.  It was the sacred wood used in Solomon's Temple; In sumeria, it represented the cosmic tree or the Tree of Life
• Pine - symbolizes uprightness, fertility, strenght of character and solitude
• Mistletoe - another popular Christmas plant, it represents the essence of life or the devine substance that gives us spiritual life
• Mint - A symbol of protection from illness, of money and love
• Spearmint - symbolizes wisdom and mental powers
• Wintergreen- a symbol of protection and healing