Superballs are made of the material vulcanized polybutadiene. When Norman Stingley synthesized this compound in 1965, he gave it the flashy name Zectron. It's the properties of this compound that make superballs super fun.
Polybutadiene molecules are essentially long chains of thousands of carbon atoms. Like rubber bands, these chains are elastic and regain their original shape if stretched. When polybutadiene is heated at high pressure in the presence of sulfur, a chemical reaction called vulcanization occurs. The reaction causes sulfur atoms to forge connections between carbon chains creating a giant network.
In other words, the sulfur atoms act as bridges. These bridges restrain the carbon strands from sliding past each other, making the material harder and more durable. Thus, when a superball bounces, its shape barely distorts. Tennis balls and racquetballs flex and compress a lot in comparison. Because the sulfur bridges limit how much a superball flexes, little energy of motion is expended to return the superball to its original shape. In fact, about 92% of a superball's energy going into a bounce remains after the bounce. That's why superballs can bounce so many times before stopping.