Stress = Force/area
Where force is in N and area is in m^2. Stress is measured in Pascals.
It is used to test how strong a material is - how much pressure a material can withstand before undergoing some sort of physical change.
Strain = extension/length
Extension = how much material has stretched (gained in length)
Length = original length
Both the extension and the length must be measured using the same units, but this measurement can be any unit (that measures length). Strain has no units as, for example, 2 metres/ 4 metres = 1/2. Strain is therefore a ratio. It is used to find out how much a material has stretched.
Young's modulus = stress/strain
It is measured in pascals as, for example, 50 pascals/2 = 25 Pascals.
Young's modulus is used to measure the stiffness of a material. It is the amount of strain a material undergos when a certain amount of stress is on it.