This means they are extruded as a continuous filament and twisted to make the thread, but they are not spun in the traditional sense. Because they are smooth, long filaments they reflect the light more and have a jewel-like look. It is also possible to have spun rayon. To spin rayon the filaments are cut into the proper lengths for the spinning machinery and then the yarn or thread is spun in the same way a cotton yarn would be. The look is softer, but since rayon dyes very well the colors are still rich.
Rayon is a cellulose fiber, as cotton is, but made from the cellulose in wood usually.
The term viscose is often interchangeable with the word rayon, although at one time there were other rayons available in addition to viscose. Again, I plan a future article on Rayon, which has an interesting history. Core spun Threads Core spun threads can be all polyester or cotton wrapped around a polyester core. If the wrap is polyester, the fibers are cut into a staple length to allow for this wrapping. Cotton is already a natural staple length.