An optical fiber is comprised of two inseparable sections - a core and its cladding.   Light propagates through the core section and the cladding provides an internal reflection boundary.  Light will propagate through the cladding, but poorly.

A multimode fiber has a large core thus allowing many light rays (modes) to propagate.

A single-mode fiber has a very small core, allowing only one mode of light to be transmitted.

Optical glass is sold based on the core/cladding dimensions.  The most common multimode fiber for fibre channel is 50/125 microns, where 50 is the size of the core (in microns) and 125 is the diameter of the cladding.  Standard multimode fibers have traditionally been covered in an orange jacket to make them readily identifiable.

In recent years, laser optimized multimode cable (OM3 and OM5) have been widely utilized for 10G applications with cabling distances up to 300m and 550m, respectively.  The cabling industry has traditionally covered these laser optimized fibers in Aqua colored jacketing to visually differentiate them from standard multimode cables.

FICON applications typically incorporate single-mode fiber.  The core size of a single-mode fiber is called the core diameter, and ranges between 8.3 microns and 10 microns.  It may be written as 8.3/125 or 9/125.    

ESCON applications use a 62.5/125, multimode cable.