I'll try not to overstate this, but next to visual hierachy, unity is probably the most important principle you can strive to achieve in your compositions...or websites, layouts, books, films, videos, dances, songs, sculptures, or anything other visual medium you are working in. 

In the visual arts, unity can be achieved through proximity, repetition and pattern, continuation, similarity, and/or rhythm of elements in a composition. In the image above, there is no unified color. Rather, all of the forms have unified shape and scale. This, compounded with their pyramidal arrangement, forces our brains to believe they all belong to the same overall construct. They don't really, but our brains have evolved and have also been trained from birth to see overall constructs and patterns. This doesn't just happen with shapes and colors: it happens with dreams, with stories, with sequences of images, with sounds, with words... it happens everywhere, all the time. 

Harmony

Harmony is very closely related to unity and is, in a way, a kind of unity. Harmony is like a state of being, achieved when the elements of a composition are unifed with one another in some way, but these elements also complement each other's unique attributes.

You can see harmony in the above image. There is certainly overall unity as the groups of multicolored hexagons all form a larger "supergroup," forming an overall pyramidal shape. But each group of 3 identically shaped and sized hexagons also fit together with radial symmetry, and therefore acheive a kind of harmony. Without this harmony between each group, the overall gestalt or unity would falter.

That's not to say that disharmony is a bad thing, or that it should be avoided. Just know that it can create a sense of disunity. Below are two quick examples of harmony and disharmony:

Repetition to create Harmony

There are many ways to achieve or break harmony, as the above examples make clear. Harmony is achieved between the square and the circle, for example, through careful superimposition of each shape.  However, another way to achieve harmony is through repetition. In the examples below, simply by repeating certain colors and shapes, and arranging purposefully, otherwise unrelated colors and shapes become related in a unified whole: