"Viscose" and "rayon" cause endless confusion, but they're effectively the same thing — rayon is the broader category, viscose the most common type. It's made by chemically processing cellulose from wood into a soft, fluid fibre.
Viscose drapes beautifully but can wrinkle, shrink, and lose strength when wet, so it often needs gentle washing. Modal and lyocell are upgraded versions engineered to be stronger and more stable.