Viscose rayon is a man-made fiber made from wood pulp though other materials may be used. It is basically modified cellulose.
Viscose is very similar to cotton and silk. (Cotton is also made of cellulose).
Viscose falls somewhere in between a natural fiber and a man-made one. The raw material for viscose is cellulose which is usually reacted with caustic soda and carbon disulfide. It is reformed as fibers by spinning this viscose solution in an acid bath. Viscose rayon has a silky appearance and feel, and also has the ability to breathe in a manner similar to cotton weaves.