With shear strain we are only concerned about the change in angles . Any time an object is deformed, shear occurs. For example, in the top row a block is deformed without changing area. It looks like the only deformation involved is compression and extension. This sort of shear is called pure shear .
Pure shear is harder to see than simple shear because there is no stationary frame of reference.
Imagine that you have planted your feet firmly along one of the diagonals of the block. As the block deforms, you see the other diagonal rotate just as you did with simple shear. To an outside observer, you also rotate, but from your perspective the two situations look identical. Even the principal strain directions look the same. In the simple shear case above, the major and minor axes of the deforming ellipse rotate clockwise as strain progresses.