Water Colour Sheet
Paper is made using a mixture of water and cellulose fibers. In artists' quality papers those fibers should be 100% cotton which makes it strong but pliable. Paper made from cotton is often referred to as "rag paper".
Less expensive papers are made using wood pulp, or sometimes a combination of cotton and other cellulose fibers. If you care about permanence or plan on using lots of scrubbing, scraping, taping, and masking techniques you should use 100% cotton paper which can take a beating.