Evergreen trees, since ancient times, have been thought to symbolize eternal life.
Before Christianity, all evergreen trees were worshipped for that reason. During
the Middles Ages, when Christianity first came to Europe, pagans who'd converted to
Christianity began calling the "tree of everlasting life" the "Paradise Tree",
symbolic of The Garden Of Eden. It was, at that time, decorated with apples.
In Germany, Christians soon began to bring "Paradise Trees" indoors during religious celebrations, especially at Christmastime. They then began to decorate the trees with wafers as well (symbolic of the Holy Eucharist). Later, other decorations began to be used, such as cookies and ribbons, and candles, lit to symbolize Christ as "the light of the world".
By the 1700s Christmas trees were popular all over Germany. They did not become popular in England, however, until the early 1800s, when Prince Consort Albert, German husband of Queen Victoria, brought them into use.
Since then the Christmas tree has continued to grow in popularity throughout the world, and in North America, it just wouldn't be Christmas without a tree hung with shiny ornaments and electric lights.