When in space two st. lines form a right angle, they are said to be perpendicular to each other. Look at the cube ABCDA1B1C1D1. The skew st. lines A1D1 and CD are perpendicular to each other. Indeed, A1D1 is perpendicular to C1D1, and C1D1 is parallel to CD.
Here are some other pairs of the perpendicular skew st. lines: A1D1 and AB, A1B1 and BC, A1B1 and AD, B1C1 and AB.