Variety: Refers to different kinds of visual content, it refers to "differences" in general. Why variety? Variety makes things interesting, without it we experience Monotony or Boredom.
  

Contrasts in composition: Contrast suggest differences between any of the elements being described. Such as size contrast, color contrast, line movement (movements) contrast, tonal contrast. Any visual element can be contrasted. Contrast contributes to clarity and variety. 
  In the picture below we can see strong contrasts in the textures, and colors, but where size is concerned, there is little contrast.

 Transition in composition: Transition is a design effect which relates to contrast listed above. But in transition there is an attempt to gradually show steps or in-between changes between two extremes of an element. For example a transition between large and small would be "medium"; a transition between black and white would be a middle tone; a transition between vertical and horizontal line direction would be a diagonal. Etc. Why use transition? Transition is a harmonizing tool. It makes "unlikes" look more harmonious. It also brings a quality of "rhythm" into the composition. Rhythm is listed below. 
  In the picture below there is a transition in the figures from reclining to kneeling, to the standing positions. This image is also a good example of grouping where the line movement of the figures takes the viewer's eye in a pyramid-like pathway.

  Rhythm in composition: Rhythm is a system where shapes or movements are similar to others in the composition. Repetition is the way rhythm occurs. Repetition means repeating line movements or similar shapes or forms throughout the picture area or "design space." Strong vertical rhythm may be called for in such subjects as trees in forest or sky scraper cityscapes. Vertical movements sometimes are used to symbolize "heavenward" or inspiration such as a church steeple. Strong horizontal rhythm might be suggested by landscape vistas, or ocean seascapes. Horizontals suggest peace and repose. 
 

In this photograph of climbers on the Devil's Tower in Wyoming we see the similarity of vertical structures that span vertically across the picture frame. Even though there is strong similarity of the rocky "ribs" there is none the less some subtle variety in each one.

Arrangement or grouping in Composition: Generally artists will form the main objects in the composition into a kind of group. If you connect the main objects in the composition with a line you will probably notice a kind of "shape" made by the object. This is shape may be triangular (very popular during the Renaissance) or some other shape. If a person is not thinking about "grouping" the result will probably be "random" or unplanned. 
 

Emphasis in composition: The term emphasis also can be called "dominance" or dominant. A feature may be dominant owing to its size, its color, or some other device which the artist uses to draw attention to the point of focus or main interest. 

The Australian Koala dominates the picture by virtue
of the size of the figure to the space. Notice though,
line movements lead the viewer in a pathway within the
picture frame. The eye is not carried outside the frame.

Pattern in composition: For a complete lesson is pattern making techniques, please click the web link called "pattern" to see a listing of pattern making systems. 

Tension in composition: Tension is a way of creating a visual "drama" within the groupings of lines and forms. Tension comes when elements are not visually comfortable with each other. 
Sometimes lines seem to be interrupted in their paths or seem to collide with other directions of movement. Tension also comes from using certain kinds of elements that produce a sense of energy. Like bright vibrating color harmonies. Some kind of lines such as angular or jagged line. 

Clarity in composition: Clarity results from having enough focus, darkness, or other emphasis of elements. The result is that they can be viewed clearly from a distance. High contrast extremes create very vivid images. Very low contrasts create unclear images. At times artists may intentionally choose to use a "high key" effect in which all forms may be nearly the same color such as the white on white designs sometimes seen in fashion photos.  

Balance in composition: Just as when two people ride a teeter totter there is a sense of balancing weights, the distribution of objects in a picture follow a kind of system. One system; formal balance approximates an even distribution on either side. (called Formal, common in Renaissance or Classic Greek images); Informal balance is general more uneven with an area made of several small forms to counter balance a single large form. A third balance system is 'Radial' balance in which all of the elements are integrated  by a reference to a central point.