Let's talk about fiber optic types and core sizes. There are three common core sizes: 9/125, 50/125, and 62.5/125. Each of those numbers stands for a measurement, and that measurement is in microns.
I quickly drew a cross-section of a fiber cable to help explain all this. The red represents the core, and that’s where you see your three measurements. The core is made of step-index graded glass. The cladding, shown here in green, has properties to keep the light inside for total internal reflection. The buffer and the jacket are all about protecting the core. Lightweight Kevlar is used to ensure this.
You may have also heard people refer to the OM rating. These are ISO standards to quantify the residual level of intermodal dispersion. This basically boils down to how fast and how far you can transmit data. Currently, OM3 is the most commonly used type.