Two of these attached to an light source is called a bifucated fiber optic system. This is the most popular type of fiber optic illuminator. A bifucated illuminator lets you adjust the angle and intensity of the light so that you can get the right amount of light and shadow. Bifucateds let you avoid glare of the specimen. A well built one is so simple to use you will wonder how you got along with out it.
Ring lights are fiber bundles with one end formed into a ring.
This provides shadow free illumination. In a lot of instances this is a real advantage. These ring lights are called annular ring lights. If you are looking at a flat specimen this may be the tool for you. Some ring lights are made with a series or small ports rather than being annular.