The actual material isn’t necessarily the be-all and end-all of a tablet case, but this factor certainly goes a long way for many users. The differences, advantages and disadvantages of materials commonly used in creating tablet cases, along with illustrations of representative examples, may be found in the Case Materials Wikipedia.
Tablet2Cases lists 15 entries on tablet cases materials, detailing each in the relevant areas of protection for the device. Materials include rugged and resistant aluminum, carbon fiber, silicon, and steel; synthetics such as neoprene, plastic, polyurethane and TPU; and more specialized matters such as bamboo, felt, leather, and animal skin.
For more information on a given material, simply click the link below for a Case Materials Wikipedia entry.