An optical fiber is a very thin, flexible glass strand that transmits light along its axis, by the process of total internal reflection.
There are two main classes of optical fibre used in telecommunications applications, Single-mode Fiber (SMF) and Multi-mode fiber (MMF)
Multi-mode Fiber is mostly used for communication over short distances, such as within a building or on a campus. Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/s over link lengths of up to 600 meters.
Single mode fiber is used for communication over very long distances. It is not so common in the datacenter environment.