A single Mode fiber is a fiber with a small core, only a few times the Wavelength of Light transmitted, that only allows one mode of light to propagate. It is commonly used with Laser sources for high speed, long distance links.
A single mode fiber is able to carry power over a greater distance than a Multimode fiber and are less susceptible to Attenuation than multimode fiber.
The equipment used with single mode fiber is more expensive than that used with multimode fiber.
Both single mode and multimode 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. This differential is what keeps the majority of light travelling down the core.