Contrapuntal motion is the movement of two or more melodic lines with respect to each other. In traditional four-part harmony, it is important that lines maintain their independence, an effect which can be achieved by the judicious use of the four types of contrapuntal motion: similar motion, parallel motion, contrary motion, and oblique motion.
This is a Bach fragment I am remembering incompletely. It demonstrates all four types of contrapuntal motion.
Similar Motion
Parallel Motion
Parallel motion is motion in the same direction, keeping the same interval between them.
Contrary Motion
Oblique Motion
Oblique motion is motion of one melodic line while the other remains at the same pitch).