Diagonal line is often referred to as the line of movement. A runner’s body or a tree bending in the wind can suggest movement. This same concept is used in works of art to imply movement as well, such as in this work by John Singleton Copley. Movement can also be suggested through implied lines as in this work by Sassetta. However, not all diagonal lines imply movement. Here is a work by Paul Cezanne that has diagonal lines that do not suggest movement.
Another use of diagonal lines is found in linear perspective. The lines line lead the viewer to the vanishing point, as in this work by Raphael.