Parts of a fiber

Core - Thin glass center of the fiber where the light travels

Cladding - Outer optical material surrounding the core that reflects the light back into the core

Buffer coating - Plastic coating that protects the fiber from damage and moisture

Optical fibers come in two types:

• Single-mode fibers
• Multi-mode fibers

Source : http://computer.howstuffworks.com/fiber-optic1.htm