The thickness of a sheet of paper is indicated by its weight, which can be measured either in grams per square meter (gsm) or pounds per ream (lb). The standard weights of machine-made paper for artists are
The "pound per ream" is a reference to what a ream of 500 sheets of paper would weigh; artists don't typically purchase 300 pounds of paper at a given time. In general, paper less than 356 gsm needs to be stretched before use to prevent it buckling or warping; paper under 190 gsm is a student grade that is too thin for anything but practice.
The gsm term is a measure for any kind of fabric, not just paper, and in all cases, a lower gsm number means a thinner fabric. For example, a light summer t-shirt measures between 130–150 gsm, while a winter-weight hoodie runs between 300–400 gsm.
Paper, like other fabrics, is made up of plant fiber from a variety of sources, such as jute, hemp, and bamboo. The most commonly used fabric in watercolor papers is cotton, and it is manufactured from cotton rags pounded into fibers and mixed with cellulose. Cellulose is wood pulp, and although it is a less expensive material compared to cotton, it contains acid, which affects the lifespan of a particular paper. "Acid-free" paper is one that is produced without cellulose, and the higher percentage of cotton ("rag content') of a paper is an indication of better quality, longer life, and higher price. A 100% cotton paper can last more than 100 years.
The cost of paper increases with the higher gsm number, and there are other considerations you should think about before choosing a paper—such as surface, machine vs. hand-made, single sheets or cylinder roll. So, if you are just starting out, you might consider buying several weights of paper, or an assortment of papers to experiment with.