ShearingTransform[, v, n] gives a TransformationFunction that represents a shear by radians along the direction of the vector v, normal to the vector n, and keeping the origin fixed.
ShearingTransform[, v, n, p] gives a shear that keeps the point p fixed, rather than the origin.
ShearingTransform works in any number of dimensions, and always gives area- or volume-preserving transformations.
In 2D, ShearingTransform turns rectangles into parallelograms. ShearingTransform[, {1, 0}, {0, 1}] effectively represents slanting by angle to the right.
In 3D, ShearingTransform does the analog of shearing a deck of cards by angle in the direction v, with the cards oriented so as to have normal vector n, and the card that goes through the point p kept fixed.