Fiber optics are plastic lines that conduct light. They have a lot of practical uses, especially in the communications and computer industries. But they can also be used for decoration.
In the past few years, trees lit with fiber optics seemed to pop out of nowhere. The prettiest ones (in my humble opinion) have the lights finely scattered throughout the branches, making the tree literally glow with a soft light. They come so that you can make tham have either a steady glow of a solid color or fade in and out through the complete rainbow of colors, making the tree seem to almost shimmer. The best picture I could find of one really doesn't do them justice: