PARALLEL MOTION, SIMILAR MOTION, OBLIQUE MOTION, CONTRARY MOTION

Even if you’ve never heard of voice leading as it applies in counterpoint, you've probably used it in your own music.

For example, when a backing vocalist sings harmony “by ear” to the lead vocal line, he or she uses contrapuntal motion.

It’s parallel motion when the lead and harmony voices move together, separated by the same type of interval, such as major and minor thirds, or major and minor sixths.
It’s similar motion when the lead and harmony voices move together, but are separated by varying types of intervals.
It’s oblique motion when one voice or part remains at the same pitch while the other moves upwards or downwards.
It’s contrary motion when one voice moves down the scale while the other moves up.