Rubber is a polymer that has the technical name polyisoprene. Rubber was originally extracted as from the sap of rubber trees, however it is also possible to synthesize artificially. Rubber molecules are long chain hydrocarbons that are normally tangled up and curled. When rubber is stretched these molecules become untangled and straighten and when the stretching force is removed the rubber returns to its original shape. This property is called elasticity. Any polymer that has this property is called an elastomer; other examples include polybutadiene, polyurethane and silicones.
Robert Hooke was the first to notice a direct link between the extension of a material and the load attached to it. With some materials, such as a metal spring, extension is directly proportional to load; This is called Hooke's Law. However, the extension of rubber is not directly proportional to load, meaning it does not obey Hooke's Law.
Rubber is initially easy to stretch as force is applied but it becomes gradually stiffer as the load is increased. Like a Hooke's Law material, rubber will return to its original shape once all of the load has been removed, however it exhibits something called mechanical hysteresis. This means that when the load is reduced gradually the extension of the rubber at that weight is greater than when this load was added. When a rubber band is stretched it gets hot, implying that it has lost some of its internal elastic energy. The difference in extension is a measure of how much of this energy has been lost.
Like all materials, rubber can only be stretched so far before it is permanently altered. This point is called the elastic limit, and past this point the rubber will not return to its original length when the load is removed. Further extension of the rubber will cause it to lose more energy as it is irreversibly deformed. This behavior is described as plastic. Ultimately, further loading will cause all of the chemical bonds at a specific point to break and the rubber will snap.