Inkjet paper substrates can be broadly divided into three main types, with some papers made from a mixture of types. Basically, the more cotton a paper has in it, the stronger and better quality it is. Cheaper papers are made from wood pulp. Plastic 'papers' are an emerging alternative, and appear in high gloss papers.
Cotton Rag papers were originally made from the remains of cotton rags - waste from the 'rag trade' fashion industry. These days plantation cotton is used. Cotton has long, strong fibres, and consequently papers made from cotton are very strong.
These papers are expensive, but the ultimate in quality. Being so strong, and naturally acid and lignin free, they are considered the most archival of papers, making them the most suitable for work in the gallery/museum, and sales context.
Wood Pulp papers are typically lower quality, weaker papers. The wood pulp fibers are shorter and less strong than cotton fibres, so this paper tears and degrades more easily. Wood pulp papers are not generally considered to be archival - for example, in the U.S. the paper Epson Archival Matte must be sold as Epson Enhanced Matte as it is not archival by Library of Congress standards.
If using a wood pulp paper, look for one that is 100% alpha cellulose wood pulp - i.e. the highest quality pulp. Wood pulp papers can be ok for proofing, but should probably be avoided in the gallery/museum and sales markets.
Plastics are also used in some 'papers' as the paper base. Older processes like Cibachrome and more modern papers like Kodak Endura Metallic use a 'paper' that has a polyester base. They are not really a paper at all and more of a plastic.
Plastics can be manufactured to be extremely smooth, making plastic based papers very suitable to the construction of super high gloss papers.