Definition:
Everyday paper, such as the stuff you put in your computer print and what magazines are printed on, is made from wood fibres (pulp), but some speciality and top-quality art papers are made from fabric fibres (rag), or with a mixture of fabric and wood fibres.
The most commonly used fibre, or rag, is cotton. Because the length of fabric fibres is longer than that of wood, rag-based papers are stronger and tend to take more 'wear', such as scrubbing with a brush.
Rag-based papers are also more archival in quality; cotton, for instance, is naturally PH-neutral.
See also: Painting on rag paper survives hurricane
