In its most basic form, as animal hide of mostly cow, leather is very thick and comprises many individual layers condensed together, each with various characteristics of their own. Broadly, these layers can be divided into two parts, the corium and the grain. Corium is the set of inner layers closer to the flesh with thinner and flexible fibers. The grain has more densely packed fibers that are sturdy and are exposed to the outside, making them very reliable and toughened materials.
Broadly, the corium is what produces suede, which the grain produces full, top, corrected and nubuck.
Image source: www.padandquill.com