The SI unit of modulus of elasticity is the pascal. Given the large values typical of many common materials, figures are often quoted in megapascals or gigapascals.
The modulus of elasticity can also be measured in other units of pressure, for example pounds per square inch (psi).
The Young's modulus allows the behavior of a material under load to be calculated. For instance, it can be used to predict the amount a wire will extend under tension, or to predict the load at which a thin column will buckle under compression. Some calculations also require the use of other material properties, such as the shear modulus, density, or Poisson's ratio.
Linear vs non-linear
For many materials, Young's modulus is a constant over a range of strains. Such materials are called linear, and are said to obey Hooke's law. Examples of linear materials include steel, carbon fiber, and glass. Rubber is a non-linear material.
Most metals and ceramics, along with many other materials, are isotropic - their mechanical properties are the same in all directions.
It is not always the case. Some materials, particularly those which are composites of two or more ingredients have a "grain" or similar mechanical structure. As a result, these anisotropic materials have different mechanical properties when load is applied in different directions. For example, carbon fiber is much stiffer (higher Young's Modulus) when loaded parallel to the fibers (along the grain). Other such materials include wood and reinforced concrete.