An SM fiber, or single Mode fiber, is a Fiber with a small core. SM fiber only allows one mode of Light to propagate, it is only a few times the Wavelength of the light signal transmitted. It is commonly used for high speed, long distance links.
A SM fiber is able to carry signals over a longer distance than a Multimode fiber. SM fiber is also less prone to Attenuation than multimode fiber.
SM fibers are also commonly called monomode fibers. SM fiber has a Core diameter of 9 µm (micrometers).
Both SM and MM fibers have an overall diameter of 125 µm from center to the outer area of Glass known as the Cladding layer. The glass in the cladding Layer has a slightly different Refractive Index than the core glass. Light is able to travel down the core because of the differential in refractive indexes.