Vellum is made from treated calfskin, while parchment can be made from the skins of sheep, goats and even deer. Parchment can also be made from calfskin. The difference between vellum and calfskin parchment lies in the process of splitting the hides, a practice specific to parchment production. Some sources strictly define vellum as being made from the uterine tissue of cattle, though this definition is not in common use.
In the case of especially thick hides, parchment makers could choose to split the hide into multiple layers. This method could produce more sheets of parchment from one hide, but the finished product would be thinner and less durable. Vellum was never split, producing a thicker and more uniform product. For this reason, vellum has a reputation for being of higher quality than parchment.
When examining historical documents, it can be difficult for scholars to distinguish between vellum and parchment. One of the most obvious distinctions is the difference in color and texture between the hair side and the flesh side of the parchment. Because vellum is not split, the side that previously formed the outer skin of the animal will have a rougher texture than the inside. It will also be whiter, because of the additional chemical processing it underwent in order to smooth it. Split parchment will have a more uniform color and texture.
In most circles, including many academic disciplines, no strong distinction is drawn between parchment and vellum. "Parchment" is a catch-all term for writing materials made from animal hide. Vellum might be referred to as "calfskin parchment" even if it has not been split. Alternatively, "vellum" is occasionally used to describe high-quality parchment whether or not it has been split or even whether it is made from calfskin.