Stress is the force applied to rocks.
Strain is the term used to describe the deformation caused by the applied forces. The strain is a change in volme or shape - or both.
Rocks behave in different ways to stress, depending on their physical properties:

competent rocks stay the same thickness when they are deformed and react in a brittle way. Sandstones, limestones and most igneous rocks are competent
incompetent rocks will vary in thickness when they are deformed, as they behave in a plastic way. Mudstones and shales are typical incompetent rocks.

Factors Affecting Stress and Strain
Three main factors which will affect the stresses applied and the resulting strains. All these factors operate together:

the higher the temperature, the more plastic the rocks will be. At high temperatures the rocks will fold, not fault, even if the original rock is competent. Cold rocks will behave as brittle materials and therefore fracture
the strength of the rocks increases with confining pressure and therefore rocks are depth are more difficult to deform. The weight of the overlying rocks causes the confining pressure
time is crucial in determining the type of deformation that occurs. If the pressure is applied for a short time, then the rocks may behave in a brittle manner, whilst pressure applied over an extended period of time can result in plastic deformation.

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Undergraduate Geology - Part 1 (Geological Structures) Notiz am Deformation of Rocks, erstellt von siobhan.quirk am 13/05/2013.