Rubber is a family of elastomeric thermoset materials. There are many different types of rubber with widely varying properties so the rubber can be tailored to the exact requirements of a specific job.

Elastomers are materials that snap back to their original shape after being deformed. There are also thermoplastic elastomers, but they are not in the rubber family.

Rubber is formed to its' final shape through a chemical reaction known as cross-linking. This reaction is initiated by heating the raw rubber compound during processing. Once the rubber is formed, the process cannot be reversed. This gives rubber the properties that make it so desirable.

There are two basic forms of rubber - dense and sponge. Dense rubber is rubber with no voids in it. Sponge rubber has been made less dense by creation of cells throughout the final part. These cells are produced by including a chemical called a "blowing agent" in the raw rubber compound. When the rubber is heated to start the cross linking reaction, the blowing agent becomes a gas and creates voids in the rubber. Sponge rubber is softer and more pliable than most dense rubbers.