In general, leather is sold in three forms:
Full-Grain leather or Top-Grain refers to hides that have not been sanded, buffed or snuffed(otherwise known as Corrected) in order to remove imperfections on the surface of the hide. Only the hair has been removed from the epidermis. The grain remains in its natural state which will allow the best fiber strength, resulting in greater durability. The natural grain also has natural breathability, resulting in greater comfort for clothing. Full grain leathers can mainly be bought as two finish types: aniline and semi-aniline.
Corrected-Grain leather is any Top-Grain leather that has had its surfaces sanded, buffed or snuffed in order to remove any imperfection on the surface due to insect bites, healed scars or brands. Most Correct leather is used to make Pigmented leather as the solid pigment helps hide the corrections or imperfections. Corrected grain leathers can mainly be bought as two finish types: semi-aniline and pigmented.
Split leather is leather that is created from the fibrous part of the hide left once the Top-Grain of the raw hide has been separated from the hide. During the splitting operation the grain and drop split are separated. Splits can are also used to create Suede.
The International Union of Leather Technologists and Chemist Societies has a full glossary of leather terms that can be found at IULTCS