Four part harmonies

You won't get a quick answer to that one, since the study of four-part harmony normally takes up 1-2 years of college music study.

In a nutshell, though, you have four simultaneous melody lines. From highest to lowest, they are labeled:

Soprano
Alto
Tenor
Bass

As the melodies move along, each one can be independent - sounding good all by itself. When they are heard together, they combine to form chords... the vertical result of the four horizontal lines, if you will.

Any two voices can be said to be moving in similar motion (both in the same direction), contrary motion (one moving up, one down), or oblique motion (one moves, the other stays put). If two voices move in the same direction by exactly the same amount, that's a subset of similar motion called parallel motion.

You can also analyze all four voices as a group - what chords they form, how they resolve.

Harmony has a lot of rules about what types of motion and resolution should be avoided... far too many to post here. The basic reference work is Walter Piston's "Harmony", although if you haven't studied any music theory or introductory harmony, you might be more comfortable easing into it with a book like William Russo's "Composing Music".