Young's modulus is a measure of a materials stiffness.
When a material is subjected to a force, if it is Hookean it will exhibit a linear region on a stress strain curve. Young's Modulus is characterised as the gradient of this initial linear section of a stress strain curve.
The linear section is highlighted in red above.
Young's modulus is key when calculating deflections of materials and also when comparing materials. For example a grade of steel and aluminium may have the same yield point (the point where they stop the linear relationship) but they will exhibit different Young's modulus. Aluminium typically has a lower young's modulus than steel, therefore it will take less force to achieve the same deflection with aluminium beam than a steel beam of the same dimensions. If both beams are deformed to the same shape then the stress in both beams will be the same.
Typical Young's modulus of materials are listed below.