Traditional vellum is a surface used for writing made from animal skin, specifically calves, lambs, or goats. The skin was processed and scraped clean, then 'polished' smooth. The side which originally had the fur on it has a slightly rougher finish to it.
In contemporary usage, the term "vellum" is used to describe paper that is relatively translucent and smooth. However, to confuse matters, the term "vellum finish" is sometimes used to describe paper with a surface texture that's got a slight tooth or roughness to it, rather than a really smooth paper. So check the actual paper, don't go by the description alone.
