Rhythm: the principle of design which indicates movement by the repetition of elements.
In music, rhythm is created with sounds that are repeated at regular intervals. People dance to a consistent rhythm and different dances are identified by the different types of rhythms produced. In art, you receive rhythm through your eyes instead of through your ears. This is called visual rhythm.
There are several repetitive elements in the example below. The windows, the sidewalk, the overhang, and even the doors create a rhythm moving horizontally. Even though each window is slightly different, they are all the same size, so a consistent rhythm is produced:
Now look at this picture:
(Broadway Boogie Woogie by Piet Mondrian)
The lines create a confusing sense of movement. They seem to go every which way, and there's no focal point. Your eye is drawn everywhere, kind of like a road map, and the smaller shapes balance the longer shapes to create an inconsistent rhythm.