One of the great difficulties in comic books is that these static images are being used to convey immense action. Each freeze frame can only show so much of the action, so artists began to use what is known as "motion lines" to help convey the idea that a character is moving at high speeds or some kind of sound or force is emanating from a weapon or explosion.
These lines will be drawn around the character or object that is in motion and will point in the direction the movement is happening. Sometimes only a few will be used, but as has been popularized by Manga, sometimes to show extreme motion, the entire background will be drawn in motion lines.
Artists are continually looking for ways to create a sense of movement in their characters and motion lines are just one way to help that along.
Pronunciation: moh-shuh n lahyns
I think the extreme motion lines in comic books are rather silly and overused.