Direction in a drawing is specified in degrees of an angle. The angle is calculated counterclockwise from the positive X axis.
The Four Primary Drawing Directions
Although the four primary drawing directions are the ones you will probably be working with the most, any angle is possible. If you disable Ortho and Angle Snap, and move an element randomly in your drawing area, there is no restriction on angles at all. Even if Ortho and Angle Snap are enabled, you can enter any angle you want in the Commander.