Durometer is an international standard for measuring the hardness of material, or relative resistance in the case of rubber. It measures ability to deflect the indentor, or the depth of indentation, and is measured with a standard rubber industry testing device that is also known as a durometer.
The hardness of the material is an important property that helps determine the suitability of a rubber product for its intended use. The components' application is critical to determining the necessary hardness. For instance, softer compounds offering less resistance may be fine for low-pressure seals where the material must conform to rough or uneven surfaces, conversely harder materials would be appropriate for high-pressure seals.
Different Shore Hardness scales are used to measure the hardness of different materials. These scales establish a common point of reference for comparing materials.
There are two standard ASTM D2240 hardness/durometer scales for rubber, Shore A (flat cone indentor) and Shore D (pointed cone indentor). High values indicate harder materials and low values indicate softer materials. Shore A is used for soft to medium hard rubber and plastics with hardness from 0 to 100, Shore D durometer is used for harder rubber and plastics and is more accurate on elastomers harder than 90 Shore A. (Note: materials may fall within one or more scales)
Shore 00: Measures light, very soft foams, sponge rubber, and gels.
Shore A: Measures soft rubber, plastics, and rubber-like elastomers.
Shore D: Measures hard rubber, plastics, and thermoplastics.
Sources:
“Sheet Rubber Handbook – Gasket and Packing Materials”, Rubber Manufacturers Association (RMA) & the Rubber Association of Canada.
http://www.smooth-on.com/Durometer-Shore-Ha/c1370/index.html
http://en.wikipedia.org/wiki/Shore_durometer
http://www.substech.com/dokuwiki/doku.php?id=shore_durometer_hardness_test